In a double-blind, double-dummy study 61 patients with structural or functional abnormalities of the urinary tract and verified infection were treated for 7 days with ofloxacin 100 mg bd or ciprofloxacin 250 mg bd by mouth. Of 62 isolated strains none was resistant to ofloxacin or ciprofloxacin in vitro. Nineteen patients (63% ) in both groups were free from infection ten days after therapy. Clinical resolution of symptoms occurred in 83% in the ofloxacin group and 68% in the ciprofloxacin group. Adverse reactions were reported in four patients (6·5% ), two with skin rash, one with gastrointestinal disturbance and one with influenza-like symptoms. This study demonstrates that ofloxacin and ciprofloxacin are effective in the treatment of complicated lower urinary tract infection.