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Resetting Microsoft Outlook's nickname cache

Outlook uses name lists called nickname caches as part of the auto-complete feature. These lists can become corrupted and need to be manually erased and rebuilt.

Microsoft Outlook 2002 and Outlook 2003 use name lists called "nickname caches" as part of the auto-complete feature. These lists allow Microsoft Outlook users to quickly call up names they've recently referred to in an abbreviated way.

For instance, let's say you previously sent an email to someone named "John Adams," and you have Microsoft Outlook's auto-complete feature turned on for address fields. Typing a "J" in an address field would present a dropdown list of names (including Mr. Adams himself). The names presented in that dropdown list are part of the Microsoft Outlook client's nickname cache.

Microsoft Outlook nicknames are generated by user behavior and aren't exclusive to a Microsoft Outlook address book. In other words, Microsoft Outlook nickname lists can contain names that aren't actually present in any address book, because the names have always been typed in manually. (If you hit Alt-K to check a name against entries in the Microsoft Outlook address book, it will not match anything in the nickname cache list.)

Normally, you can easily delete unwanted names from the Microsoft Outlook nickname cache. You simply use the up or down arrows to select a nickname from the list when it's visible, and then hit the Delete key. However, if the Microsoft Outlook nickname list gets corrupted, this won't work, and the nickname list itself may no longer come up.

In such cases, the entire nickname list must be erased and rebuilt:

  1. Close any running copies of Microsoft Outlook.

  2. Open Windows Explorer.

  3. Make sure hidden files and folders are visible in Windows Explorer by navigating to Tools -> Folder Options -> Advanced Settings, and select "Show hidden files and folders."

  4. Look in the directory \Documents and Settings\<username>\Application Data\Microsoft Outlook for any files with the .NK2 extension. These are the nickname caches. (If they're not visible there, you may need to search the system for any files that match that extension.)

  5. Rename the file to something else. For example, if the file name is Outlook.NK2, you could rename it to Outlook.NK2.bak.

  6. Restart Microsoft Outlook. A new Microsoft Outlook nickname cache should now be rebuilt.

About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.


You should note that the nickname list can only contain 1,000 entries. If you exceed 1,000 names, the list will shut down.

It would be better to delete the names you don't use (by using the delete key) as you use Microsoft Outlook.
—Andy P.


The correct/complete path of .NK2 file extension is \Documents and Settings\<username>\Application Data\Microsoft\Outlook\ .

If you have multiple profiles configured, then you will have multiple .NK2 files prefixing the profile name.

You can always save a copy of the .NK2 file on your desktop. If you switch to a new desktop or laptop, then you can copy the file in the same location on the new desktop or laptop, but make sure the profile name is same as the .NK2 file name.

Happy patching...
—Athif K.


These are very good supplements to the tip -- thanks for writing in!
—Serdar Yegulalp, tip author

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Related information from

  • Tip: How to automatically clear the forms cache in Microsoft Outlook
  • Learning Center: The Microsoft Outlook Toolbox
  • Reference Center: Microsoft Outlook tips and resources

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