No, that won't work for me... There has to be a postback, because my tree is dynamic, it is filled in "lazy" mode on server-side. So I can't check the checkboxes on the client-side - they may not exist there at all...

Do you know how can I update the update panel on the client-side? I know that there is some way using PageRequestManager... But I can't find how exactly this works. This is the only thing I need to solve the issue now...