The efficacy of a single-dose (4 tablets) trimethoprim-sulfamethoxazole (TMP-SMX) was compared with that of a 3-day and 10-day treatment with TMP-SMX, 2 tablets twice daily, in 464 female out-patients with symptoms denoting acute, uncomplicated urinary tract infection (UTI). 321 patients (70%) had significant bacteriuria. Treatment effect could be assessed in 279 women. Comparable results were obtained with the 3 regimens 2 and 6 weeks after treatment. Eradication of the initial organism occurred in 96% with single-dose, in 96-94% with a 3-day, and in 98% with a 10-day course. The incidence of adverse reactions was significantly greater in patients treated with a 10-day (28%) than in those treated with a single-dose (5%), or 3-day (9%) regimen (p < 0.01). This study suggests that short treatment regimens for uncomplicated UTI in women are as effective as and cause fewer side-effects than the conventional 10-day chemotherapy.