Event handler for optionlist-selection

[Claus Bogner]

Hi,
 
I would like to implement a event for selection of a dropdown-list.
When the user selects an element in optionlist A the content
of the optionlist B should be refreshed.
 
Example: 
 
optionlist A:   List of countries (Germany, France, Austria)
optionlist B:   List of cities in the selected country.
 
Does anyone know how I could realize that behaviour? Is tried
to find some example for that but with no success.
 
Thanks in advance,
Claus