Vista - form input can't hold variables?

Hello
I have this problem, i can't figure out a solution:

<input type="text" id="num_scheda" name="numeroscheda" size="20"
value="<%= pippo %>" maxlength="9" />

when page load the text input correctly show the contents of variable
pippo, but if the user delete the input text, and then reload the page
i expect that variable pippo is shown again, but this doesn't happen,
i mean pippo is set
(if i make a response.write pippo it will show) but is not displayed
on the input form..
Any ideas?
Thanx in advance