put this formula in B1 on that other sheet
and copy it down...
=IF(COUNTIF(Sheet1!A$1:A$500,A1),
MATCH(A1,Sheet1!A$1:A$500,0),"")

Presumably you mean:

=IF(COUNTIF(Sheet1!A$1:A$50000,A1),
INDEX(Sheet1!B$1:B$50000,MATCH(A1,Sheet1!A$1:A$500 00,0)),"")

Or

=IF(COUNTIF(Sheet1!A$1:A$50000,A1),
VLOOKUP(A1,Sheet1!A$1:B$50000,2,0),"")

I can't believe I forgot to included it... I was going for the INDEX formula
version. Thanks for catching that.

Rick Rothstein (MVP - Excel)