OBJECTIVE To compare trospium chloride (TCl), a quaternary ammonium derivative with atropine-like effects and predominantly antispasmodic activity, with oxybutynin (Oxy) in terms of efficacy and adverse effects. METHODS In a randomized, double-blind, multicentre trial, 95 patients with spinal cord injuries and detrusor hyper-reflexia were studied. Treatment consisted of three doses per day over a 2 week period, with either Oxy (5 mg three times daily) or with TCl (20 mg twice daily) with an additional placebo at midday. The results were evaluated with regard to changes in objective (urodynamic) data and subjective symptoms as well as the incidence/severity of adverse effects. RESULTS With both drugs there was a significant increase in maximum bladder capacity, a significant decrease in maximum voiding detrusor pressure and a significant increase in compliance and residual urine; there were no statistically significant differences between the treatment groups. The percentage of patients who reported severe dryness of the mouth was considerably lower (4%) in those receiving TCl 2 x 20 mg/day than in those receiving Oxy (23%) 3 x 5 mg/day. Withdrawal from treatment was also less frequent in those receiving TCl (6%) than in those receiving Oxy (16%). CONCLUSIONS Trospium chloride and oxybutynin, judged in terms of objective urodynamic parameters, are of substantially equal value as parasympathetic antagonists. However, assessment of tolerance in terms of adverse drug effects showed that TCl had certain advantages.