Prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP) serum levels were measured in 117 patients with prostatic adenocarcinoma, in 9 patients with prostatic hyperplasia and in 14 patients with other malignancies to compare the clinical usefulness of the PSA and PAP levels. PSA was elevated (PSA+) in 14 of 18 untreated patients (78%) with prostatic cancer. PAP was elevated (PAP+) only in 3 of these untreated cases (17%). Also in previously treated patients PSA was more often positive than PAP. PSA was positive in 40 of the 99 treated patients (40%), PAP was elevated only in 21 cases (21%). There was a significantly (P less than 0.001) higher tendency towards elevated PSA in the prostatic cancer patients: 32 (27%) patients with PSA+ and PAP- compared with only 2 cases (2%) with PAP+ and PSA-. The PSA+/PAP- patients were analyzed further. In seven of them the PSA level also returned to its normal level after orchiectomy or/and radiotherapy. In two patients the PSA levels indicated tumor progression earlier than PAP, their PAP levels did not rise until bone metastasizing was evident. There were also progressive disease in some patients evidenced only by increased PSA levels. In addition to cancer patients the PSA level was increased in three (30%) of the prostatic hyperplasia patients. It was also elevated in three patients with other malignancies. However, these three patients also had prostatic hyperplasia and the increase in the PSA level is considered more likely to be due to that.(ABSTRACT TRUNCATED AT 250 WORDS)