dynamic drop-down list

Hi..

Your help is appreciated. I want to display only choices in drop-down filed depend on the previous filed entered.

example:

region: west, east, south, north
Now when user selects as below:
1. west   ---- then only Los Angeles, San Jose appear in Center drop down field.
1. East   ---- then only New Jersy, New York appear in Center drop down field.
1. South   ---- then only Atlanta, Georgia appear in Center drop down field.
1. North   ---- then only Canada, Seatile appear in Center drop down field.

Could you please help me how do i get this done.

appreciated .

thanks,