Use the "subtotal" function instead of sum. It has 2 parameters for input: a
function # and the range to be considered. The function # is used to give
count, average, sum, and other functions, and it works on all cells unless
you add 100 to it. The function for summing is 9, but if you use 109 it will
ignore hidden cells. Note that this function does not do well summing
horizontally, and will not ignore hidden columns if you use it that way.
When used on a vertical range it will ignore the hidden rows. See Excel help
for the subtotal function for more info.
Good luck,
AC