The clinical and bacteriological efficacy of a 3-day course of pivmecillinam, 200 mg three times daily, was compared with that of a 7-day course of cephalexin, 250 mg four times daily, in 216 patients with a bacteriologically confirmed, acute, uncomplicated, urinary tract infection. Both treatments were similarly effective. Clinical cure or improvement was obtained in 95.3% of patients given pivmecillinam and in 93.6% of patients given cephalexin. Bacteriological success was achieved in 89.7 and 81.7% patients taking pivmecillinam or cephalexin, respectively. Eradication rates for Escherichia coli were 90.1% for pivmecillinam and 80.6% for cephalexin. Both treatments were well tolerated. This study has confirmed that a 3-day course of pivmecillinam is effective and well tolerated in uncomplicated cystitis.