The efficacy and adverse event profile of a single 3.25 g dose of co-amoxiclav as treatment of acute uncomplicated lower urinary tract infection in women was compared with that of co-trimoxazole 960 mg bd for 7 days in a prospective, randomized, double-blind multicentre clinical trial. Of the 666 patients enrolled, 279 (144 in the co-amoxiclav group and 135 in the co-trimoxazole group) were eligible for evaluation of clinical and bacteriological responses. At the follow-up assessment 42 days after study entry, the successful clinical response rate was 73.8% for patients who received co-amoxiclav, compared with 85.1% for patients given co-trimoxazole (P < or = 0.05); the corresponding rates for successful bacteriological response were 64.1% and 79.6% (P < or = 0.05). Both treatment regimens were well-tolerated, with 15% of patients in the co amoxiclav group and 12% of patients in the co-trimoxazole group reporting adverse events (P > or = 0.05). The adverse event profiles for the two groups differed, gastrointestinal disturbances predominating amongst patients who received co-amoxiclav and rashes being commonest amongst those given co-trimoxazole.