Forty-two patients with multiple myeloma that responded to initial treatment were observed for at least 6 months during a 2-year period. Eleven deaths occurred, seven of these being associated with hypercalcemia. None of these patients developed renal insufficiency before the serum calcium level rose, although kidney function deteriorated later in five of them. In those patients who are susceptible, hypercalcemia appears to be a good early indicator of relapse in multiple myeloma.