diff --git a/lib/builder.js b/lib/builder.js
index 0dee247..f062dbb 100644
--- a/lib/builder.js
+++ b/lib/builder.js
@@ -20,6 +20,7 @@ function ICSFileBuilder() {
   this.NEWLINE_CHAR = '\r\n';
   this.throwError = false;
   this.ignoreTZIDMismatch = true;
+  this.busystatus = 'FREE';
 
 }
 
@@ -107,6 +108,7 @@ ICSFileBuilder.prototype.toString = function () {
     if (_event.description) lines.push('DESCRIPTION:' + _escape(_event.description));
     if (_event.url) lines.push('URL;VALUE=URI:' + _event.url);
     if (_event.status) lines.push('STATUS:' + _event.status.toUpperCase());
+    if (_event.busystatus) lines.push('X-MICROSOFT-CDO-BUSYSTATUS:' + _event.busystatus.toUpperCase())
     if (_event.organizer) lines.push('ORGANIZER;' + (!!_event.organizer.sentBy ? ('SENT-BY="MAILTO:' + _event.organizer.sentBy + '":') : '') + 'CN="' + _event.organizer.name.replace(/"/g, '\\"') + '":mailto:' + _event.organizer.email);
     for (var ac = 0; ac < _event.attendees.length; ac++) {
       if (_event.attendees[ac] instanceof Error) {
@@ -184,6 +186,13 @@ ICSFileBuilder.prototype._checkAndBuildEventObject = function (rawEvent) {
       'CONFIRMED',
       'TENTATIVE',
       'CANCELLED'
+    ],
+    allowedBusyStatuses = [
+      "FREE",
+      "WORKINGELSEWHERE",
+      "TENTATIVE",
+      "BUSY",
+      "OOF"
     ];
 
   if (!rawEvent || typeof rawEvent !== 'object') {
@@ -325,6 +334,12 @@ ICSFileBuilder.prototype._checkAndBuildEventObject = function (rawEvent) {
   }
   event.status = rawEvent.status;
 
+  // Busy Status
+  if (rawEvent.busystatus && allowedBusyStatuses.indexOf(rawEvent.busystatus.toUpperCase()) === -1) {
+    return new Error('event.busystatus must be one of the following: ' + allowedBusyStatuses.join(', '));
+  }
+  event.busystatus = rawEvent.busystatus;
+
   // URL
   event.url = rawEvent.url || null;