Rifampicin plus trimethoprim (rifaprim) was used to treat 20 patients with chronic prostatitis in exacerbation: 11 received 2 tablets at bedtime for 15 days followed by 1 tablet at bedtime for another 105 days, and 9 received 1 tablet in the morning and 2 tablets at bedtime for 15 days, then 2 tablets at bedtime for 15 days followed by 1 tablet at bedtime for another 90 days. All patients had an enlarged tender prostate and all but 2 were symptomatic. In 10 patients previous treatment, including co-trimoxazole in 5, had failed. Cultures of the expressed prostatic secretions yielded Staphylococcus aureus in 17 patients and gram-negative micro-organisms in 3. At the end of treatment 6 of 11 patients given the lower dosage were cured clinically and bacteriologically compared to 8 of 9 given the higher dosage. After 2 to 3 years of following 5 of 9 patients in the first group and all 7 in the second group had not suffered relapse. From our study it is evident that rifaprim is a potent drug in the treatment of chronic prostatitis caused mainly by Staphylococcus aureus. A promptness of therapeutic response and the rate of cure at the end of treatment as well as after at least 2 years of followup favor the higher drug dosage.