Objectives. To evaluate the behavior of bone alkaline phosphatase (BAP) after androgen deprivation therapy in patients with prostate cancer. Methods. BAP was determined in the serum of 35 patients with nonmetastatic prostate cancer before maximal androgen blockade. During 5 years of follow-up, BAP was determined annually. A subset of 36 patients who underwent radical prostatectomy as the only treatment for prostate cancer was selected as a control group. Results. The mean pretreatment BAP serum concentration was similar in both groups (10.8 and 10.4 ng/mL, P >0.05). However, after 5 years, the mean BAP serum concentration increased 64.8% in the study group (from 10.8 to 17.6 ng/mL, P < 0.001); it remained stable in the control group (from 10.4 to 10.4 ng/mL). The increase of serum BAP in patients receiving androgen deprivation was 32.4% during the first year (from 10.8 to 14.3 ng/ml), 16.7% during the second (from 14.3 to 16.1 ng/mL), 7.4% during the third (from 16.1 to 16.9 ng/mL), 5.5% during the fourth (from 16.9 to 17.3 ng/mL), and 2.8% during the fifth year (from 17.3 to 17.6 ng/mL). Conclusions. Androgen deprivation produces an increase in the BAP serum concentration. A major increase seems to be produced during the first year of follow-up and thereafter this increase is reduced around 50% annually. Copyright © 2002 Elsevier Science Inc.