Thirty-six patients, 9 males and 27 females, average age 13.2 years, undergoing posterior fusion and segmental instrumentation surgery for idiopathic adolescent scoliosis were studied for abnormalities of platelet count (PC), prothrombin time (PT) and partial thromboplastin time (PTT). Intraoperative and immediate postoperative values were obtained at 2, 4, 6, 12, 24, and 48 hours. All patients showed diminution of platelet counts during the study period, but mean values were reduced below normals only at 4 hours after operation. The PT was significantly elevated in all patients intraoperatively and for the first 24 hours postoperatively, with return to normal by 48 hours. The PTT levels were essentially unaffected at any time.