Prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP) were determined in the serum of 69 patients with clinical T3/T4M0 prostatic cancer before staging lymphadenectomy. In principle, high-dose radiotherapy was given only to patients of pathological N0 category. Seventeen patients had a prelymphadenectomy PSA level below the normal upper reference limit (10 micrograms/l) and only 3 of them had pelvic lymph node metastases. Fifteen of 52 patients with a preoperative PSA level > or = 10 micrograms/l were of N0 category. Only 8 of the 41 evaluable patients had PAP values above the normal range, and 6 of these 8 patients had pelvic lymph node metastases. Preoperative PSA values, but not preoperative PAP levels, assist the clinician in predicting regional lymph node metastases in patients with clinical T3/T4M0 prostatic cancer. Two-thirds of the patients with T3/T4 tumours and PSA values between 10 and 50 micrograms/l have regional lymph node metastases. About 80% of the patients with PSA levels < 10 micrograms/l belong to the N0 category. About 75% of the patients with PSA > 50 micrograms/l have N+ disease. Taking into account the individual preoperative PSA values, the indication for preradiotherapy staging lymphadenectomy should be balanced between the chance of demonstrating N+ disease, the expected postoperative morbidity and the benefit for the patient found to be of N0 category.