The hypnotic effects of 7.5 mg zopiclone, as well as its unwanted and residual effects, were compared with those of 5 mg nitrazepam in a double-blind, randomized, multicenter, parallel group study. After an initial, 7-day, placebo wash-out period, insomniac out-patients under the care of general practitioners received either 7.5 mg zopiclone or 5 mg nitrazepam for 6 weeks. Everyday, the patients filled in a diary (analogue scales for sleep parameters and condition during daytime); a sleep questionnaire was filled in at baseline and at the end of active treatment period; every 2 weeks, a somatic complaint check-list inventory and a mood rating scale were filled in and psychomotor tests performed; and at the end of the study, a global evaluation of efficacy and acceptability was given by the investigator. Clinical laboratory tests were performed before and after the active treatment period. From the diary, sleep onset latency, as well as sleep quality, were similarly improved by both drugs throughout the whole study. From the sleep questionnaire, all sleep parameters measured were improved at the end of the 6-week treatment period in both groups. No statistical differences in the various psychomotor tests were observed between the two treatment groups, and a significant improvement in the working ability of patients was noted with both drugs. Some significant differences were observed in the mood rating scale and the somatic complaint check list, probably related to differences in pharmacokinetics of the two drugs.