Sixty consecutive hospitalized patients with complicated urinary tract infections were randomly assigned in a ratio of two patients to 200 mg i.v. ofloxacin per day and one patient to 3 x 4000 mg piperacillin. The duration of treatment was seven days. Ten patients were excluded from the study because of primary resistance of the pathogens to the respective drug (6 of the 40 ofloxacin patients and four of the 20 piperacillin patients). Gram-negative bacteria were isolated in 64% and Staphylococcus species in 21%. A complete eradication of the organisms was found three to four days after the end of therapy in 75% of the ofloxacin group and in 73% of the piperacillin patients. At the three to four weeks follow-up reinfections were observed in three patients of each group. There were no statistical differences in the results. Therapy was discontinued because of adverse reactions (nausea, vomiting) in one patient from each group. The study shows that it is possible to treat complicated urinary tract infections just as successfully with 200 mg i.v. ofloxacin per day as with 3 x 4000 mg piperacillin per day.