Quantitative bone histology was carried out in five osteosclerotic adults. The bone was extremely hard in all patients, and open biopsy was usually required. One patient, aged 18 years, presented with hypoplastic anemia, and the most probable explanation for the osteosclerosis is a marrow stem cell defect leading to defective osteoclasts. Another had the dominant form of osteopetrosis. Her bone contained cartilage remnants, and there were many large, morphologically abnormal osteoclasts, which lacked normal cytoplasmic acid phosphatase activity. The third patient had chronic renal failure and osteomalacia; here the increased bone mass might have resulted from an inability of normal osteoclasts to resorb bone, due to the surface coating of osteoid, though an earlier increase of bone formation cannot be excluded. The fourth patient, who suffered from systemic mastocytosis, had high turnover bone, with greatly, increased bone formation. The fifth patient, with fluorosis of bone, also had increased bone formation and resorption, the process being much more pronounced in the head of her pathologically fractured femur than it was in the iliac crest. In this patient some osteoclasts had reduced acid phosphatase activity and long cytoplasmic extensions, both changes similar to those observed in diphosphonate-treated animals. Very diverse processes can result in the increased cancellous bone mass producing the radiographic appearance of diffuse osteosclerosis.