Dynamic listbox

[Vik]

I need to add/remove the items to/from the dropdownlists/listboxes on an
..aspx page. I tried the following code in JavaScript:
document.getElementById("ListBox1").add(new Option("5","5"));
The code adds the new item for a moment (I can see it in the listbox and can
retrieve it by the code) but then the new item disappears from the listbox.

How can I fix it?

Thanks.

 

[Vik]

The problem was in that I run this code by a button that caused the
postback.

Vik