Tartrate-resistant acid phosphatase is contained in multinucleated giant cells of giant cell tumour of bone (GCT) and chondroblastoma (CBL) as well as in osteo-clasts. Yet few studies have so far been done regarding serum acid phosphatase (AcP) level in patients of GCT or CBL. The purpose of this study is to elucidate the clinical significance of serum AcP as a tumour marker for GCT and CBL. Serum AcP value was examined in nine GCT patients and three CBL patients before and after surgery. In the GCT cases, serum AcP values before surgery were high in five cases. They were 14.0 IU/L, 68.7 IU/L, 45.9 IU/L, 21.9 and 31.3 IU/L (normal value; 7.1–12.6 IU/L). They decreased after surgery to 7.7 IU/L (55% of the preoperative value), 8.2 IU/L (12%), 7.8 IU/L (17%), 6.1 IU/L (28%) and 10.0 IU/L (32%), respectively. Serum AcP values before surgery were within normal limits in the remaining four GCT cases. Even in these four cases, postoperative serum AcP level was lower than the preoperative level. Postoperative/preoperative AcP ratios in these four cases were 67%, 80%, 69% and 76%. In the CBL cases, serum AcP values were high in all cases. They were 15.1 IU/L, 13.1 IU/L and 13.7 IU/L. They decreased after surgery to 10.3 IU/L (68% of the pre-operative value), 10.2 IU/L (78%) and 9.7 IU/L (71%), respectively, all within normal limits. Therefore, it is concluded that serum AcP is a useful tumour marker for GCT and CBL in diagnosing the tumour as well as in evaluating the efficacy of treatment.