PURPOSE: To evaluate the utility of preoperative nidus localization guided by computed tomography(CT). MATERIALS AND METHODS: In the light of clinical, radiographic, and imaging findings, osteoid osteoma was suspected in all ten patients studied. An osteoid osteoma of the tibia was present in seven cases, and of the femur in three. During the 24 hours preceeding surgery, a small hole was drilled directly over the nidus with a 15gauge Ostycut bone biopsy needle, using CT for control and guidance. The hole and needle tract were stained with Gentian violet and GE 9800 and Hi Speed Advantage CT (GE) were used to obtain both 2D and 3D images. RESULTS: Each osteoid osteoma was in the cortex of the bone, nidi ranged in size from 10x6x6 to 40x7x9mm and the sclerotic area around each varied in size between 150x18x14 and 50x15x12mm. Marking was successful on the first attempt in eight cases, on the second attempt in one case and on the third attempt in one case. The average time per marking attempt was 15 minutes. In the operating field, stained marking on the cortex was easily found inall cases. With the marking it was possible to excise only as much of the bone as necessary and none of the ten patients suffered complications either during or after the procedure. The diagnosis of osteoid osteoma was histopathologically confirmed in all patients. CONCLUSION: CT guided nidus marking of an osteoid osteoma assured precise localization during surgery ; bone resection was therefore minimal and the time required for surgery was reduced. The procedure is a simple, safe, effective method for precise and easy nidus excision.