There is now considerable evidence showing the benefits of single dose antibacterial treatment for uncomplicated urinary tract infections. If single dose therapy is to become widely used it is necessary to clarify the minimum effective dose of the most efficient drugs. This paper reports three trials in women presenting in general practice with bacterial cystitis. In each trial the patients were randomly allocated to either a five day course of oral co-trimoxazole (CTM) or to doxycycline 300 mg orally, cefuroxime 1.5 g intramuscularly or pivmecillinam 600 mg orally. Thirty-eight of 45 patients treated with doxycycline were cured, compared with 44 or 45 treated with CTM. Fourteen of 20 women given cefuroxime were cured, compared with 19 of 20 prescribed CTM. Twenty-three of 30 women treated with pivmecillinam were cured, compared with all 30 given CTM. None of these three drugs, when administered as a single dose, was as effective as a single 1.92 g or 2.88 g dose of CTM used in our previous studies in domiciliary practice. These studies confirmed, however, that single dose therapy was well tolerated, preferred by the patients and side effects were minimal.