Flow cytometry can differentiate benign from malignant lesions of the prostate through deoxyribonucleic acid distribution analysis. A method has been developed that permits simultaneous cytometric determination of deoxyribonucleic acid and acid phosphatase activity in the cell cycle compartments of prostatic biopsy specimens. Histograms of prostatic carcinoma reveal higher acid phosphatase activity and greater deoxyribonucleic acid content in the S and S + G2/M populations than the histograms representing benign lesions. This compartmental difference may have prognostic usefulness.