Even though it's designed for mass-production, you could use mailmerge for this. Simply do the normal mailmerge setup then, each time you want to produce a letter, open the mailmerge main document and 'finish' the merge. You would need to change the format of the worksheet used for the mailmerge, though, so that it has each 'ouput' field in different columns on the same row.

The alternative is to use linking. However, unless you unlink the linked data each time you create a new letter, any saved document of that letter will retain the links - which will reset to the current worksheet values anytime you open the document.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]