Transiliac bone biopsies from 136 untreated pagetic patients, taken from both pagetic (72 cases) and nonpagetic bone (64 cases), were quantitatively analyzed. Tetracycline double labeling analysis demonstrated that osteosclerosis of pagetic bone results from an overproduction of woven bone by the osteoblasts. Increased bone cell populations appeared as the consequence of an increased birthrate of the basic multicellular units. In 45% of patients biopsied in nonpagetic bone, there were prints of an increased bone remodeling attributed to a hyperparathyroidism secondary to the high calcium demand of pagetic bones.