Hi Zvonimir Mavretić,
Which version of RadListBox are you using? I have just tested with Q2 Service pack 1 (this is 2009.2 826) and the issue is not reproducible there. I suggest that you upgrade to the latest version of RadControls.
You can verify the that the issue is fixed on our online demos.
However, if upgrade to a newer version is not applicable we will try to provide you with a fix. Please, open a support ticket if this is the case.
the Telerik team