Variable Number of Drop Down List with Dynamic Content

Hello,
 
I am working on a webform that will have a variable number of drop down lists.  Each drop down list will generate a new drop down list when an item has been selected and that item has children.  What would be the best way to do this? Ideally, I would like to avoid a postback.  Also, I'm working with asp.net 3.0.
 
Thank you,
Ryan

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