A prospective, randomised trial was undertaken to compare the efficacy of ciprofloxacin and netilmicin for the treatment of acute pyelonephritis. Forty-three patients were enrolled and 34 (29 women) completed the protocol. Fifteen of 17 patients treated with ciprofloxacin and 15 of 17 treated with netilmicin were cured. All patients were treated for five days. One patient relapsed after ciprofloxacin and another had a reinfection, while two relapsed after netilmicin. Five of six patients with a urinary tract abnormality were cured. Side effects were generally mild and rapidly reversible. Patients treated with ciprofloxacin spent a mean of 3.7 days in hospital compared with 5.3 days for those treated with netilmicin. The difference in duration of hospital stay was statistically significant (p less than 0.01). Both drugs proved highly effective and safe for the treatment of severe acute pyelonephritis.