Can you clarify a bit please - as far as I know, Excel lets you have around
255 columns - are you saying that you need more columns than that, or are
you saying that you are encountering some other limitation on the number of
fields in your Word document? (If the latter, can you describe it?) Or are
you saying that merge cannot "see" all the fields in your Excel sheet?

If the answer is that you need more than 255 columns, the only way you will
be likely to get Word merge to work is to store your data in a delimited
text format instead - (most other formats also impose a limitation on the
number of fields). However, even that may not work with the current version
of Word, particularly if the number of fields is very large.

Peter Jamieson

Must be some document!

Your only chance is going to be if you can split the mailmerge main document
into a number of separate documents that draw their mergefields from a
single spreadsheet.  You would then have to execute the merge for each main
document to a new document and then use a macro to combine the corresponding
sections from each of these documents.