A direct and efficient method for evaluation of the error probability of opticalheterodyne receivers in the presence of phase noise is presented. A closed form expression for the statistics of the decision variable, including photodetector shot noise and thermal noise from electronic circuitry, is derived. The analysis assumes simple integrating filters in the receiver and is based on a power series expansion of the filtered phase noise. The errorprobability is calculated using a saddle point approximation which is numerically simple and gives accurate results. The optimal prefilter bandwidth for bestphase noise rejection is easily determined.