diff --git a/lib/open3.rb b/lib/open3.rb
index 5ff1012..ef0cdcd 100644
--- a/lib/open3.rb
+++ b/lib/open3.rb
@@ -223,6 +223,7 @@ class << self
   # <code>opts[:stdin_data]</code> and <code>opts[:binmode]</code>.  See Process.spawn.
   #
   # If <code>opts[:stdin_data]</code> is specified, it is sent to the command's standard input.
+  # It can be given as a String or an IO.
   #
   # If <code>opts[:binmode]</code> is true, internal pipes are set to binary mode.
   #
@@ -248,13 +249,13 @@ class << self
   #   # But if the image is stored in a DB or generated by the gnuplot Open3.capture2 example,
   #   # Open3.capture3 should be considered.
   #   #
-  #   image = File.read("/usr/share/openclipart/png/animals/mammals/sheep-md-v0.1.png", :binmode=>true)
+  #   image = File.open("/usr/share/openclipart/png/animals/mammals/sheep-md-v0.1.png", :binmode=>true)
   #   thumbnail, err, s = Open3.capture3("convert -thumbnail 80 png:- png:-", :stdin_data=>image, :binmode=>true)
   #   if s.success?
   #     STDOUT.binmode; print thumbnail
   #   end
   #
-  def capture3(*cmd, stdin_data: '', binmode: false, **opts)
+  def capture3(*cmd, stdin_data: nil, binmode: false, **opts)
     popen3(*cmd, opts) {|i, o, e, t|
       if binmode
         i.binmode
@@ -263,9 +264,15 @@ def capture3(*cmd, stdin_data: '', binmode: false, **opts)
       end
       out_reader = Thread.new { o.read }
       err_reader = Thread.new { e.read }
-      begin
-        i.write stdin_data
-      rescue Errno::EPIPE
+      if stdin_data
+        begin
+          if stdin_data.respond_to?(:read)
+            IO.copy_stream(stdin_data, i)
+          else
+            i.write stdin_data
+          end
+        rescue Errno::EPIPE
+        end
       end
       i.close
       [out_reader.value, err_reader.value, t.value]
@@ -281,6 +288,7 @@ def capture3(*cmd, stdin_data: '', binmode: false, **opts)
   # <code>opts[:stdin_data]</code> and <code>opts[:binmode]</code>.  See Process.spawn.
   #
   # If <code>opts[:stdin_data]</code> is specified, it is sent to the command's standard input.
+  # It can be given as a String or an IO.
   #
   # If <code>opts[:binmode]</code> is true, internal pipes are set to binary mode.
   #
@@ -311,7 +319,11 @@ def capture2(*cmd, stdin_data: nil, binmode: false, **opts)
       out_reader = Thread.new { o.read }
       if stdin_data
         begin
-          i.write stdin_data
+          if stdin_data.respond_to?(:read)
+            IO.copy_stream(stdin_data, i)
+          else
+            i.write stdin_data
+          end
         rescue Errno::EPIPE
         end
       end
@@ -329,6 +341,7 @@ def capture2(*cmd, stdin_data: nil, binmode: false, **opts)
   # <code>opts[:stdin_data]</code> and <code>opts[:binmode]</code>.  See Process.spawn.
   #
   # If <code>opts[:stdin_data]</code> is specified, it is sent to the command's standard input.
+  # It can be given as a String or an IO.
   #
   # If <code>opts[:binmode]</code> is true, internal pipes are set to binary mode.
   #
@@ -346,7 +359,11 @@ def capture2e(*cmd, stdin_data: nil, binmode: false, **opts)
       outerr_reader = Thread.new { oe.read }
       if stdin_data
         begin
-          i.write stdin_data
+          if stdin_data.respond_to?(:read)
+            IO.copy_stream(stdin_data, i)
+          else
+            i.write stdin_data
+          end
         rescue Errno::EPIPE
         end
       end
diff --git a/test/test_open3.rb b/test/test_open3.rb
index d52ab6a..0015c57 100644
--- a/test/test_open3.rb
+++ b/test/test_open3.rb
@@ -2,6 +2,7 @@
 
 require 'test/unit'
 require 'open3'
+require 'stringio'
 
 class TestOpen3 < Test::Unit::TestCase
   RUBY = EnvUtil.rubybin
@@ -162,32 +163,70 @@ def test_capture3_flip
     assert(s.success?)
   end
 
+  def test_capture3_stdin_string
+    o, e, s = Open3.capture3(RUBY, '-e', 'print STDIN.read', :stdin_data=>"z"*(1024*1024))
+    assert_equal("z"*(1024*1024), o)
+    assert_equal("", e)
+    assert(s.success?)
+  end
+
+  def test_capture3_stdin_io
+    o, e, s = Open3.capture3(RUBY, '-e', 'print STDIN.read', :stdin_data=>StringIO.new("z"*(1024*1024)))
+    assert_equal("z"*(1024*1024), o)
+    assert_equal("", e)
+    assert(s.success?)
+  end
+
+  def test_capture3_stdin_closed
+    o, e, s = Open3.capture3(RUBY, '-e', '', :stdin_data=>"z"*(1024*1024))
+    assert_equal("", o)
+    assert_equal("", e)
+    assert(s.success?)
+  end
+
   def test_capture2
     o, s = Open3.capture2(RUBY, '-e', 'i=STDIN.read; print i+"o"', :stdin_data=>"i")
     assert_equal("io", o)
     assert(s.success?)
   end
 
+  def test_capture2_stdin_string
+    o, s = Open3.capture2(RUBY, '-e', 'print STDIN.read', :stdin_data=>"z"*(1024*1024))
+    assert_equal("z"*(1024*1024), o)
+    assert(s.success?)
+  end
+
+  def test_capture2_stdin_io
+    o, s = Open3.capture2(RUBY, '-e', 'print STDIN.read', :stdin_data=>StringIO.new("z"*(1024*1024)))
+    assert_equal("z"*(1024*1024), o)
+    assert(s.success?)
+  end
+
+  def test_capture2_stdin_closed
+    o, s = Open3.capture2(RUBY, '-e', '', :stdin_data=>"z"*(1024*1024))
+    assert_equal("", o)
+    assert(s.success?)
+  end
+
   def test_capture2e
     oe, s = Open3.capture2e(RUBY, '-e', 'i=STDIN.read; print i+"o"; STDOUT.flush; STDERR.print i+"e"', :stdin_data=>"i")
     assert_equal("ioie", oe)
     assert(s.success?)
   end
 
-  def test_capture3_stdin_data
-    o, e, s = Open3.capture3(RUBY, '-e', '', :stdin_data=>"z"*(1024*1024))
-    assert_equal("", o)
-    assert_equal("", e)
+  def test_capture2e_stdin_string
+    oe, s = Open3.capture2e(RUBY, '-e', 'print STDIN.read', :stdin_data=>"z"*(1024*1024))
+    assert_equal("z"*(1024*1024), oe)
     assert(s.success?)
   end
 
-  def test_capture2_stdin_data
-    o, s = Open3.capture2(RUBY, '-e', '', :stdin_data=>"z"*(1024*1024))
-    assert_equal("", o)
+  def test_capture2e_stdin_io
+    oe, s = Open3.capture2e(RUBY, '-e', 'print STDIN.read', :stdin_data=>StringIO.new("z"*(1024*1024)))
+    assert_equal("z"*(1024*1024), oe)
     assert(s.success?)
   end
 
-  def test_capture2e_stdin_data
+  def test_capture2e_stdin_closed
     oe, s = Open3.capture2e(RUBY, '-e', '', :stdin_data=>"z"*(1024*1024))
     assert_equal("", oe)
     assert(s.success?)