  We study the existence of periodic solutions of the non--autonomous periodic
Lyness' recurrence u_{n+2}=(a_n+u_{n+1})/u_n, where {a_n} is a cycle with
positive values a,b and with positive initial conditions. It is known that for
a=b=1 all the sequences generated by this recurrence are 5-periodic. We prove
that for each pair (a,b) different from (1,1) there are infinitely many initial
conditions giving rise to periodic sequences, and that the family of
recurrences have almost all the even periods. If a is not equal to b, then any
odd period, except 1, appears.
