In an open randomized study pivmecillinam (Selexid; CAS 32886-97-8) was studied by general practitioners in 345 female patients with uncomplicated acute cystitis. Out of the bacteriologically evaluated 299 patients 151 patients were treated for three days with two tablets of pivmecillinam 200 mg t.i.d. and 148 patients for seven days with one tablet t.i.d. There were no significant differences in the bacteriological effect between the two regimens. In the 3-day group 91% and 88% were cured at the first and the second control; in the 7-day group 94% and 95%, respectively. There was no significant difference in the total clinical effect, either. Adverse reactions, usually gastrointestinal disturbances, occurred in 10% of the 3-day group and in 11% of the 7-day group (N.S.). Pivmecillinam treatment in acute cystitis in women was equally effective whether given for three or seven days, with the same total frequency of adverse reactions for the two regimens.