We have shown that prostate specific antigen (PSA) levels can be as readily obtained from voided urine as from serum samples. This procedure was found to give stable and reproducible results. PSA analyses were performed on voided urine collected from 42 patients with benign prostatic hypertrophy (BPH), 27 with stage D2 prostate cancer and 57 after radical prostatectomy. The 42 BPH samples had a mean urinary PSA level of 216 ng./ml., which did not correlate with estimated prostate size. For 4 of 5 patients with stage D2 disease who presented before hormonal therapy urinary PSA levels were greater than 50 ng./ml. For 22 stage D2 patients seen after initiation of hormonal therapy the majority had low urinary PSA levels. After initiation of hormonal therapy in most cases low urinary PSA levels were found in conjunction with high serum PSA values. However, in other cases we found high urinary PSA with low serum PSA levels. Of 43 patients who underwent radical prostatectomy for stages A to C disease it was noteworthy that 77% had elevated urinary PSA levels, while only 33% had elevated serum levels. Therefore, close to 80% of these patients have prostate tissue remaining locally after this operation.