The effectiveness of oxybutynin in the treatment of primary enuresis was evaluated in a double-blind study. A total of 30 children (25 boys, five girls), at least 6 years of age, with primary enuresis and no daytime incontinence or history of other urinary tract problems were selected at random from an enuresis clinic population. The study was explained to the families and they were told how to keep records of nocturnal bed-wetting episodes and side effects. The patients were treated with a 10 mg of oxybutynin at suppertime for 28 days. Before or after the treatment period, all children received an identical placebo for 4 weeks. Two-sided paired t tests were used to compare frequency of nocturnal enuresis. Frequency during the drug regimen did not differ significantly from that during the placebo study. There were no differences in findings between boys and girls or between children who had previously taken imipramine and those who had not. The study showed no evidence that oxybutynin is effective in treating primary enuresis.