As urinary tract infections in immunosuppressed renal transplant patients present a major therapeutic problem for clinicians in charge of renal units, the efficacy of the antibiotic ciprofloxacin in such cases was tested in this study. Twenty-six patients, 16 women and 10 men, aged 20 to 56 years, who developed uninary tract infection (UTI) from 6 months to 10 years after renal transplantation were included in the study. Of these patients, 20 (77%) showed cystitis and/or prostatitis and 6 (23%) clinical symptomatology of acute or recurrent pyelonephritis. Patients with obstructive uropathy were excluded. Urine culture was positive for E. coli in 16/26 patients (61.5%) and for proteus mirabilis, klebsiella, staphylococcus aureus in 10/26 (38.5%). All patients were given ciprofloxacin 250 mg x 2 daily for 10 days and the results of the treatment were compared to those of 60 nontransplant patients (controls) with UTI. Fourteen patients (54%) were completely cured and 10 (38%) showed improvement, while the respective results in the controls were 68% (41/60) and 28%. Relapses occurred in two patients, one in each group. Serious side effects were not observed. It is concluded that ciprofloxacin is an effective and safe drug for the treatment of UTI in renal transplant patients.