This report contains a statistical review of 559 cases of lesions forming hard tissues that were diagnosed by the departments of Clinical Pathophysiology and of Pathology at Tokyo Dental College from 1966 to 2001. Sixteen kinds of lesions which were related to the formation of bone, cartilage or cementum were analysed: osteoma, osteo-chondroma, chondroma, osteoid osteoma, osteoblastoma, ossifying fibroma, cemento-ossifying fibroma, cementifying fibroma, so-called cementoma, cementoblastoma, gigantiform cementoma, periapical cemental dysplasia, osteosarcoma, chondrosarcoma, fibro-osseous lesion, and fibrous dysplasia of bone. The most common lesion was osteoma (203 cases). There is a marked tendency for this condition to occur in females (201 males cases and 358 female cases). The patients' ages ranged from 3 to 84 years, and the mean was 40.1 years old. Lesions with hard tissue formation were observed most frequently in the third decade and in the mandibular molar region.