OBJECTIVE To confirm that the bone isoenzyme of alkaline phosphatase (ALP) contributes significantly to the increased ALP in normal pregnancy, and to determine the gestational age at which the increase occurs. METHODS Cross-sectional determinations of serum total, bone and placental ALP were carried out in 67 normal pregnant women. The results were compared with those of 18 normal age-matched non-pregnant controls using Student's t-test. RESULTS At both 31-32 weeks and 38 weeks of gestation, serum activities of bone ALP were increased significantly (P < 0.05 and P < 0.001, respectively) when compared with controls. The percentage contribution of bone ALP to the total was higher at 31-32 weeks and lower at 38 weeks when compared with the contribution from placental ALP. CONCLUSIONS Knowledge of time of elevation of serum bone ALP activity in normal pregnancy will be helpful in the interpretation of elevated serum total ALP activity during pregnancy.