Microscopic foci of prostatitis may induce prostate-specific antigen (PSA) increase. PSA reduction after antibiotics might identify those patients in whom biopsy can be avoided. Ninety-nine patients received ciprofloxacin for 3 weeks, of whom 59 showed PSA reduction. Histology detected small foci of prostatitis in 65% of cases. Carcinoma was found in 40 and 20.3% of patients with unchanged or decreased PSA, respectively (P = 0.03). No cancer was detected if PSA decreased below 4 ng/ml or more than 70%. Biopsy can be postponed, with a low risk of missing a cancer, if PSA decreases more than 70% or below 4 ng/ml.