Excel Lookup Function

I'm having a problem using the lookup function; I have a workbook set up with two worksheets. The second worksheet is one very large spreadsheet that is set up with all of the budget data by inventory class, along with two dozen columns of information about each class. I've set up the first worksheet as a "form" that an analyst can use to key in an inventory class. My desire is to be able to key in the inventory class and have several other fields on this first worksheet populate from the second worksheet using the LOOKUP function. I use a command such as "=lookup(C11,base2001!a4:d322)" to populate a field from the BASE2001 worksheet, where cell C11 on the first worksheet matches those inventory classes in column A of the second worksheet, and the desire is to populate this cell in the first worksheet from those values contained in column D in the second worksheet. I can get this to work, but it seems to work somewhat randomly - on some classes it works accurately, but in some other cases it either can't find a match or returns values from another row! Any ideas? Sorry this is so long!

Tim Pedersen