This monograph records clinical studies and the roentgenographic, histologic and chemical study of the bones of 2 patients dying with osteopetrosis. A previously nonrecorded feature was the discovery of conical structures in practically, all the bones. A technic is described to demonstrate these cones by roentgen ray examination. The conical structures are considered to be the result of the absence of or partial lack of resorption of the cartilage and endochondrial or membrane bone. The retardation of bone resorption is accompanied by generalized fibrosis of the marrow with hypoplasia of the blood vessels and resultant aplasia of the marrow and extramedullary hematopoiesis. The disease is characterized as a recessive hereditary morbid condition, the onset coinciding with the first appearance of ossification, about the second fetal mouth. The bibliography includes one hundred and sixty items.