The safety and efficacy of zopiclone (7.5 mg) and pentobarbitone (100 mg) were compared in 60 adult outpatients suffering from insomnia. The patients were randomly assigned to one of the two treatment groups, and the medication was taken at bedtime for 16 days. Zopiclone and pentobarbitone, compared with placebo, improved sleep onset, duration of sleep, number of night time awakenings, and quality of sleep. Zopiclone was superior to pentobarbitone with regard to sleep quality, judgement of therapy, and condition in the morning. Side effects were reported in each treatment group, but were less frequent in the zopiclone group (p < 0.005). Zopiclone is a useful drug for the treatment of sleep disturbances, not only because of its efficacy, but also because of its tolerability.