diff --git a/Readme.md b/Readme.md
index c000ada..a41d08a 100644
--- a/Readme.md
+++ b/Readme.md
@@ -73,6 +73,11 @@ pluralize.plural('irregular') //=> "irregulars"
 pluralize.addIrregularRule('irregular', 'regular')
 pluralize.plural('irregular') //=> "regular"
 
+// Example of new irregular rule preserving casing, e.g. "ID" -> "IDs":
+pluralize.plural('ID') //=> "IDS"
+pluralize.addIrregularRule('ID', 'IDs', { preserveCasing: true })
+pluralize.plural('ID') //=> "IDs"
+
 // Example of uncountable rule (rules without singular/plural in context):
 pluralize.plural('paper') //=> "papers"
 pluralize.addUncountableRule('paper')
diff --git a/pluralize.js b/pluralize.js
index 65dfcd5..1bccc35 100644
--- a/pluralize.js
+++ b/pluralize.js
@@ -17,11 +17,11 @@
 })(this, function () {
   // Rule storage - pluralize and singularize need to be run sequentially,
   // while other rules can be optimized using an object for instant lookups.
-  var pluralRules = [];
-  var singularRules = [];
-  var uncountables = {};
-  var irregularPlurals = {};
-  var irregularSingles = {};
+  const pluralRules = [];
+  const singularRules = [];
+  const uncountables = {};
+  const irregularPlurals = {};
+  const irregularSingles = {};
 
   /**
    * Sanitize a pluralization rule to a usable regular expression.
@@ -86,7 +86,7 @@
    */
   function replace (word, rule) {
     return word.replace(rule[0], function (match, index) {
-      var result = interpolate(rule[1], arguments);
+      const result = interpolate(rule[1], arguments);
 
       if (match === '') {
         return restoreCase(word[index - 1], result);
@@ -110,11 +110,11 @@
       return word;
     }
 
-    var len = rules.length;
+    let len = rules.length;
 
     // Iterate over the sanitization rules and use the first one to match.
     while (len--) {
-      var rule = rules[len];
+      const rule = rules[len];
 
       if (rule[0].test(word)) return replace(word, rule);
     }
@@ -132,8 +132,13 @@
    */
   function replaceWord (replaceMap, keepMap, rules) {
     return function (word) {
+      // If replaceMap has the exact casing, we preserve the casing
+      if (replaceMap.hasOwnProperty(word)) {
+        return replaceMap[word];
+      }
+
       // Get the correct token and case restoration functions.
-      var token = word.toLowerCase();
+      const token = word.toLowerCase();
 
       // Check against the keep object map.
       if (keepMap.hasOwnProperty(token)) {
@@ -155,7 +160,7 @@
    */
   function checkWord (replaceMap, keepMap, rules, bool) {
     return function (word) {
-      var token = word.toLowerCase();
+      const token = word.toLowerCase();
 
       if (keepMap.hasOwnProperty(token)) return true;
       if (replaceMap.hasOwnProperty(token)) return false;
@@ -173,8 +178,9 @@
    * @return {string}
    */
   function pluralize (word, count, inclusive) {
-    var pluralized = count === 1
-      ? pluralize.singular(word) : pluralize.plural(word);
+    const pluralized = count === 1
+      ? pluralize.singular(word)
+      : pluralize.plural(word);
 
     return (inclusive ? count + ' ' : '') + pluralized;
   }
@@ -257,9 +263,13 @@
    * @param {string} single
    * @param {string} plural
    */
-  pluralize.addIrregularRule = function (single, plural) {
-    plural = plural.toLowerCase();
-    single = single.toLowerCase();
+  pluralize.addIrregularRule = function (single, plural, options) {
+    const preserveCasing = !!(options && options.preserveCasing);
+
+    if (!preserveCasing) {
+      plural = plural.toLowerCase();
+      single = single.toLowerCase();
+    }
 
     irregularSingles[single] = plural;
     irregularPlurals[plural] = single;
diff --git a/test.js b/test.js
index fc84d37..1b104e1 100644
--- a/test.js
+++ b/test.js
@@ -1,14 +1,14 @@
 /* global describe, it */
 
-var expect = require('chai').expect;
-var pluralize = require('./');
+const expect = require('chai').expect;
+const pluralize = require('./');
 
 /**
  * Standard singular/plural matches.
  *
  * @type {Array}
  */
-var BASIC_TESTS = [
+const BASIC_TESTS = [
   // Uncountables.
   ['firmware', 'firmware'],
   ['fish', 'fish'],
@@ -677,7 +677,7 @@ var BASIC_TESTS = [
  *
  * @type {Array}
  */
-var SINGULAR_TESTS = [
+const SINGULAR_TESTS = [
   ['dingo', 'dingos'],
   ['mango', 'mangoes'],
   ['echo', 'echos'],
@@ -692,7 +692,7 @@ var SINGULAR_TESTS = [
  *
  * @type {Array}
  */
-var PLURAL_TESTS = [
+const PLURAL_TESTS = [
   ['plateaux', 'plateaux'],
   ['axis', 'axes'],
   ['basis', 'bases'],
@@ -801,6 +801,14 @@ describe('pluralize', function () {
       expect(pluralize('irregular')).to.equal('regular');
     });
 
+    it('should allow new irregular words with preserved casing', function () {
+      expect(pluralize('ID')).to.equal('IDS');
+
+      pluralize.addIrregularRule('ID', 'IDs', { preserveCasing: true });
+
+      expect(pluralize('ID')).to.equal('IDs');
+    });
+
     it('should allow new plural matching rules', function () {
       expect(pluralize.plural('regex')).to.equal('regexes');