Try putting Check in brackets?

Or CkNum isn't a numeric field in the table?

Too few parameters. Expected 1

This means Access doesn't recognise one field in your SQL statement.
It is either mis-spelled or doesn't exist.

I've tried [Check]=CkNum. Check is a Long Integer field in the table. I've used this code: MsgBox CkNum + CkNum   to check that it is a number and the MsgBox doubles the check number value--so it is numeric. And if I hard-code the check number like: Check=301  then it works fine. But when I try to use a variable, it doesn't.

Try this.

CurrentDb.Execute "UPDATE InvoiceT SET Check=" & CkNum & " WHERE InvoiceID=" & [TempVars]![InvoiceID]

Wow. Kevin. Yes. That makes it work! Thanks for your input.

You're welcome.

Remember: the variable should be outside the quotes.