Osteolytic defects and osteoporosis are common in myeloma, while sclerotic lesions of bone are rare. Eighteen patients with increased bone density have been described in the literature and five patients are presented in this report. Diffuse increase in skeletal density, similar to that seen in the myelofibrosis-myelosclerosis syndrome, occurred in two patients, and progressive multiple focal areas of sclerosis with splenomegaly in a third. Two patients had solitary areas of sclerosis. Although there was increased cortical and trabecular bone, osteoblastic activity was normal on histological sections. Whether the sclerosis was due to new bone formation or interference with bone resorptive processes could not be determined. Patients with polycythemia, myelofibrosis and myelosclerosis have been found to have, or later develop, myeloma. This has led to the suggestion that myeloma be included among the myeloproliferative disorders. At present the evidence for this interrelationship is the frequency of the association of these diseases.