An efficient method is presented which implements the start-up from no
ise in numerical simulations of free-electron lasers in the steady sta
te. The method is related to the one described by Penman and McNeil O
ptics Comm. 90 (1992) 82 !, but has the advantage that it does not nee
d the use of a random number generator, so it lacks a statistical spre
ad in the simulation results.