The present study is the first to investigate the effect of the antidepressant nefazodone on sleep in patients with primary (psychophysiological) insomnia. Following baseline assessment of sleep (polysomnography and subjective sleep parameters), 32 patients received initially 100 mg nefazodone in a single dose at bedtime; according to efficacy and tolerability, the dose could be increased up to 400 mg. Polysomnography and assessment of subjective sleep parameters were repeated after 4 weeks' administration. 12 patients dropped out, 11 of them due to lack of efficiency or intolerable side effects. In 20 patients who completed, the authors observed a lengthened sleep onset latency, decreases in stage 1 and slow wave sleep, and increases in stages 2 and REM under nefazodone. Subjective measures of sleep mirrored a clearer improvement: there was a significant reduction of the PSQI total score and all subscores except sleep latency. We suppose that the dose range chosen was too high for this patient population, thus accounting for the high proportion of dropouts and the partly unfavorable effects on objective sleep parameters. For a definite evaluation of the possible role of nefazodone in the treatment of primary (psychophysiological) insomnia, double-blind, placebo-controlled, randomized studies with lower doses are needed.