'Order by' question

Hi, I have a MySQL varchar column which I wish to display. Most of the entries are numbers eg 1, 2, 3 etc etc. But some are numbers and letters eg 34A, 34B, 35A. When I display the column I am able to sort by number using the "order by column_name+0" command, as in: $result = mysql_query("SELECT id, house, pen, date_format(start,'%M %D, %Y') as start, date_format(end,'%M %D, %Y') as end, details FROM animalhouse WHERE house='old' ORDER BY pen+0") or die (mysql_error()); However, it doesn't sort the letters properly, so I get 34B, 34A, 34C etc etc. Is there a simple way for me to make it sort by number first and then by letter? Thanks Justin