diff --git a/ChangeLog b/ChangeLog
index 27d4bd3fdf..3b7611663b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,13 @@ Sun Oct 22 16:47:56 2006  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 	* strnig.c (rb_str_new4): keep shared string untainted when orignal
 	  string is tainted.  fixed: [ruby-dev:29672]
 
+Sun Oct 22 07:55:11 2006  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* string.c (rb_str_upcase, rb_str_downcase, rb_str_downcase,
+	  rb_str_upcase_bang, rb_str_downcase_bang, rb_str_swapcase_bang):
+	  add RDoc description that case conversion to be effective only
+	  in ASCII region.
+
 Sat Oct 21 17:50:40 2006  Akinori MUSHA  <knu@iDaemons.org>
 
 	* ext/digest/lib/digest.rb: Follow the framework updates.
diff --git a/string.c b/string.c
index 39bae6d63d..529c375b4f 100644
--- a/string.c
+++ b/string.c
@@ -2711,6 +2711,7 @@ rb_str_dump(VALUE str)
  *  
  *  Upcases the contents of <i>str</i>, returning <code>nil</code> if no changes
  *  were made.
+ *  Note: case replacement is effective only in ASCII region.
  */
 
 static VALUE
@@ -2744,6 +2745,7 @@ rb_str_upcase_bang(VALUE str)
  *  Returns a copy of <i>str</i> with all lowercase letters replaced with their
  *  uppercase counterparts. The operation is locale insensitive---only
  *  characters ``a'' to ``z'' are affected.
+ *  Note: case replacement is effective only in ASCII region.
  *     
  *     "hEllO".upcase   #=> "HELLO"
  */
@@ -2763,6 +2765,7 @@ rb_str_upcase(VALUE str)
  *  
  *  Downcases the contents of <i>str</i>, returning <code>nil</code> if no
  *  changes were made.
+ *  Note: case replacement is effective only in ASCII region.
  */
 
 static VALUE
@@ -2796,6 +2799,7 @@ rb_str_downcase_bang(VALUE str)
  *  Returns a copy of <i>str</i> with all uppercase letters replaced with their
  *  lowercase counterparts. The operation is locale insensitive---only
  *  characters ``A'' to ``Z'' are affected.
+ *  Note: case replacement is effective only in ASCII region.
  *     
  *     "hEllO".downcase   #=> "hello"
  */
@@ -2815,6 +2819,7 @@ rb_str_downcase(VALUE str)
  *  
  *  Modifies <i>str</i> by converting the first character to uppercase and the
  *  remainder to lowercase. Returns <code>nil</code> if no changes are made.
+ *  Note: case conversion is effective only in ASCII region.
  *     
  *     a = "hello"
  *     a.capitalize!   #=> "Hello"
@@ -2855,6 +2860,7 @@ rb_str_capitalize_bang(VALUE str)
  *  
  *  Returns a copy of <i>str</i> with the first character converted to uppercase
  *  and the remainder to lowercase.
+ *  Note: case conversion is effective only in ASCII region.
  *     
  *     "hello".capitalize    #=> "Hello"
  *     "HELLO".capitalize    #=> "Hello"
@@ -2876,6 +2882,7 @@ rb_str_capitalize(VALUE str)
  *  
  *  Equivalent to <code>String#swapcase</code>, but modifies the receiver in
  *  place, returning <i>str</i>, or <code>nil</code> if no changes were made.
+ *  Note: case conversion is effective only in ASCII region.
  */
 
 static VALUE
@@ -2912,6 +2919,7 @@ rb_str_swapcase_bang(VALUE str)
  *  
  *  Returns a copy of <i>str</i> with uppercase alphabetic characters converted
  *  to lowercase and lowercase characters converted to uppercase.
+ *  Note: case conversion is effective only in ASCII region.
  *     
  *     "Hello".swapcase          #=> "hELLO"
  *     "cYbEr_PuNk11".swapcase   #=> "CyBeR_pUnK11"