Twenty-one patients, two months to seventy-eight years old, with clinically diagnosed osteopetrosis showed radiographic and histological variations in the formation of radiodense skeletal tissue at all ages. Pathologically the abnormal tissue was composed of both lamellar bone and calcified cartilage. We deduced that the abnormality or abnormalities of osteoclast function that cause the observed changes vary in severity from time to time in individual patients. Periods of remission in the less severely affected patients apparently had permitted the formation of bone-marrow spaces and allowed hematopoietic tissue to form in these patients. The patients who died of the disease did so because of anemia or its resulting complications. Some children with a lethal form of the disease had rickets and epiphyseal fractures in addition to the osteopetrosis. Focal areas of osteomalacia were observed in two specimens from adults. Traumatic and stress fractures were frequent complications at all ages.