I'm going to assume that your listbox and button are members of the same class. So, all you need to do is declare a class level variable, like so:
Class myForm
Private Shared myThreadDelegate As New ThreadStart(AddressOf ThreadWork.DoWork)
Private Shared myThread As New Thread(myThreadDelegate)
End Class [color=green]'myForm[/color]
You can then access your thread from any sub or function within the class, including your listbox and button event handlers. However, if your thread creation code resides in another class you'll need take divil's advice and pass references as needed.
Good Luck
-CL