Does anyone know if it is a law in Illinois that all new accounts must be started with number 101 on their checks. We have a new customer that is insisting that we start their checks with a much higher number.

The Illinois Consumer Deposit Account Act discusses starting numbers. Look at 205 ILCS 605/0.01

Please start your post with the name of the state you are asking about. In fact, if you don't mind, will you please edit your post's subject. It wastes valuable time for compliance officers outside of IL to look at a post that they have no information about.