Complete resection is usually impossible for fibrous dysplasia (FD) involving the cranial base. Incomplete resection could be followed by regrowth of FD, but there is no method for indicating disease progress. Serum alkaline phosphatase (ALP) is significantly high in patients with FD. The authors investigate the relationship between ALP, progress of FD, and age at surgery. 18 patients with craniofacial FD were separated into 3 groups: Group A, complete resection; Group B, incomplete resection followed by regrowth of FD; and Group C, incomplete resection but no regrowth of FD. Medical records and CT scans were reviewed retrospectively. ALP levels were obtained preoperatively, postoperatively and every year during follow-up. The relation between ALP and regrowth and that between age at surgery and regrowth were investigated. There was no recurrence in Group A (n=4). Regrowth in Group B (n=7) was preceded by an abrupt increase in ALP. In Group C (n=7), no regrowth was observed and ALP was maintained within the normal range. 6 patients (85%) in Group B and 2 (28%) in Group C were under 17 years old. The results revealed that the level of postoperative serum ALP could be a reliable marker for predicting the progress of craniofacial FD.