One hundred and eighty-four women, with acute uncomplicated cystitis, received a 3-day course of pivmecillinam comprising an initial 400 mg (two tablets) dose, followed by 200 mg every 8 hours; a total of ten tablets. A satisfactory clinical response was achieved in 91% of patients. Bacteriological success was observed in 94% of sixty-eight patients with a proven infection. Side-effects were reported in sixteen patients (8.4%). Two patients ceased therapy prematurely. Pivmecillinam did not select out resistant Gram-negative organisms. A trend towards reinfection with Gram-positive cocci was observed. The possible significance of reinfection with different organisms is discussed.