  The dimensionally regularized massless on-shell double box Feynman diagram
with powers of propagators equal to one is analytically evaluated for general
values of the Mandelstam variables s and t. An explicit result is expressed
either in terms of polylogarithms Li_a(-t/s), up to a=4, and generalized
polylogarithms S_{a,b}(-t/s), with a=1,2 and b=2, or in terms of these
functions depending on the inverse ratio, s/t.
