A retrospective study has been made on the interrelationship of serum bone alkaline phosphatase (BAP), measured by the Ostase-RIA, and prostate-specific antigen (PSA) in 156 patients with M0 and M1 prostate cancer. BAP is a more sensitive and more specific method of determining osteoblast activity than total alkaline phosphatase (TAP). The main difference between these two assays is seen when the TAP is in the range of normal to twice-normal. BAP shows a low intraindividual variation in M0 disease, and was within normal limits in 18 patients following radical prostatectomy with a PSA < 0.1 ng/ml. A raised BAP was observed in 86.4% of M1 disease at diagnosis before treatment. The change of BAP was concordant with PSA in 69% of 49 cases of M1 disease, although there are marked differences in the rates of change of the two markers. A nadir of PSA < or = 10 ng/ml after androgen blockade in M1 disease was associated with a high probability of a normal BAP.