In a one-year follow-up study, short-term therapy with ciprofloxacin proved to be beneficial in the treatment of chronic bacterial prostatitis, particularly in cases of Escherichia coli prostatitis. Ciprofloxacin therapy of E. coli prostatitis was a complete success in five of 12 patients, a probable success in two, and in one patient the outcome cannot be judged. Ciprofloxacin treatment failed in three men, and therapy was discontinued in one patient because of side effects. Enterobacter aerogenes prostatitis was cured. Ciprofloxacin treatment was insufficient in three patients with Streptococcus faecalis prostatitis and in one patient with Pseudomonas aeruginosa prostatitis.