Intendi una variabile javascript?

In tal caso:
codice:
<script type="text/javascript">
  var pagiframe = pagina_variabile.htm

document.write("<iframe frameborder='0' width='215' height='115' src="+pagiframe+" marginheight='0' marginwidth='0' 
scrolling='yes' name='Box' allowtransparency='yes'></iframe>")
</script>