The presence of epithelioid osteoblasts, lace- or sheet-like osteoid production, and a permeative pattern of tumor growth in osteoblastomas is thought to be associated with an aggressive clinical behaviour. This study assessed the prognostic significance of these and other histologic parameters by analyzing a large group of cases. Histologic material obtained from 55 patients who had osteoblastoma diagnosed and treated at Memorial Sloan-Kettering Cancer Center was analyzed. Additionally, the radiographic images were studied and the lesions were radiologically staged as stage 1 (quiescent), stage 2 (active), or stage 3 (aggressive). Epithelioid osteoblasts were detected in 14% of the cases without any mitotic activity. Lace- or sheet-like osteoid was present in 36% of the cases studied. A permeative pattern of tumor growth was present in 15% of lesions in all but one arising in the short tubular or large flat bones. Thirty-four percent of the lesion were in stage 1, 48% in stage 2, and 17% in stage 3. All stage 1 tumors involved long tubular bones, whereas all stage 3 tumors arose in the short tubular or flat bones. Local recurrence was noted in 16% of patients, all of whom had stage 2 lesions. One patient with a vertebral tumor eventually died with persistent disease. No association between the histologic features and disease outcome was demonstrated. The clinically aggressive behavior of osteoblastoma is not related to particular histologic features, but rather to the skeletal location. Mitotic activity is not present in osteoblasts in the osteoblastoma.