Hey Henk (Sounds dutch to me),

it is very hard if not impossible to use a validation list with data from
other work sheets.

If it is somehow possible you should try to get the information in the same
sheet as the validationlist.

I myself use =IF('Sheet1'!A17="";"";'Sheet1'!A17) copy it till row 3000
somthing, and then hide the collum.

Works almost perfect.

Additional information can be required using a VLOOK option
=VLOOKUP(D17;'Sheet1'!A17:L41;8;FALSE)

Have fun, Groeten

Jasper