A total of 538 patients from 45 different general practice centres across the UK was admitted to an open study and randomized to one of the following treatment groups: nitrofurantoin modified release (MR) 100 mg bd, trimethoprim 200 mg bd or co-trimoxazole 960 mg bd. Each patient received seven days of medication. Clinical cure, defined as relief from symptoms at visit 2, occurred in 87.2% of the patients treated with nitrofurantoin MR, 84.5% of the co-trimoxazole group and 86.5% of the trimethoprim group. The bacteriological cure rate for nitrofurantoin MR was comparable to co-trimoxazole at 82.3% and 83.2%, respectively, with trimethoprim the lowest at 76.8%. Whilst the cure rate for Escherichia coli infection was similar, 81.5% cured with nitrofurantoin MR, 82.5% with co-trimoxazole and 78.4% by trimethoprim, for non-E. coli pathogens nitrofurantoin MR was equivalent to co-trimoxazole with 86.7% cure but higher than trimethoprim at 72.0%. In-vitro sensitivity to all pathogens isolated at baseline was very high for nitrofurantoin at 96.1%, significantly higher than either co-trimoxazole or trimethoprim at 87.5% (P < 0.01). The test drugs were equally well tolerated with 28 patients (15.7%) reporting adverse events with nitrofurantoin MR, 28 (15.5%) with co-trimoxazole and 28 (15.6%) with trimethoprim. However, nitrofurantoin MR showed fewer patients with drug-related adverse events (5.6%) as judged by the investigator, compared to co-trimoxazole (8.8%) or trimethoprim (7.3%). (ABSTRACT TRUNCATED AT 250 WORDS)