In an open study, 172 male and female adult patients with acute uncomplicated bacterial cystitis were randomly allocated to three treatment groups. Two groups received brodimoprim 200 mg tablets as follows: a single dose of two 200 mg tablets on day 1, followed by one tablet per day on days 2 and 3 (58 patients); or a single daily dose of two tablets, for 2 days (63 patients). The third group received a single dose of pefloxacin, as two 400 mg tablets, for 1 day (51 patients). Complete urinalysis, sediment and urine culture examinations were carried out before treatment and 10 days after the last dose. Evaluation also comprised, at the time of enrolment and 48 h after the last dose, measurement of corporal temperature and assessment of symptoms (dysuria, pollakiuria, strangury, suprapubic pain, burning sensation during urination and urgency) on a 4-point scale. The eradication rate for the pathogen concerned was 98.3% and 96.7% in the groups receiving brodimoprim for 3 and 2 days, respectively, and 92.8% in the pefloxacin group (between-group comparison n.s.). There was significant regression of symptoms (P < 0.001) in the three groups (between-groups comparison n.s.). Mainly gastrointestinal adverse events occurred in 3 patients receiving brodimoprim for 2 days and in 4 patients from each of the other two groups.