Measurements of total body myeloma cell number and osteoclast activating factor (OAF) production by bone marrow myeloma cells in vitro were made in 33 patients with plasma cell myeloma. There was a highly significant correlation (P < 0.001) between the amount of OAF produced by bone marrow myeloma cells and the extent of skeletal destruction. Serial studies indicated reduction of OAF production with remission induction and high OAF production at the time of relapse with new bone destruction. Although hypercalcaemia occurred only in patients with the most extensive bone lesions, there was no direct relationship between OAF production and the serum calcium concentration. These data suggest that measurement of OAF production in multiple myeloma may predict those patients at risk for development of extensive skeletal destruction due to this neoplasm.