Oxybutynin hydrochloride (3 mg) was compared with placebo by randomised, double-blind crossover trial in 53 females with idiopathic detrusor instability. Symptoms were cured or markedly improved in 60% of patients on oxybutynin and 2.3% on placebo. During the first treatment period, oxybutynin reduced the frequency of voiding by 35%, compared with 9% for placebo. Oxybutynin gave a significantly greater improvement than placebo in volume at the first desire to void (70 ml increase versus 7.7 ml), maximum filling-phase detrusor pressure (17 cm H2O reduction versus no benefit) and cystometric capacity (104 ml increase versus 7.0 ml). A marked oxybutynin carry-over effect was seen during the second treatment period. Side effects from the 3 mg dose of oxybutynin caused 7.5% of subjects to discontinue therapy.