Seventeen patients with plasma cell leukemia (more than 20% plasma cells in the peripheral blood and an absolute plasma cell content of at least 2,000/cu mm) are described. In 12 patients, the leukemic phase appeared before myeloma was diagnosed, whereas in 5, myeloma was diagnosed first. The group with plasma cell leukemia had a much higher incidence of hepatosplenomegaly, lower initial hemoglobin values, more renal insufficiency, more leukocytosis, and more circulating plasma cells than did a group of patients with multiple myeloma who did not develop plasma cell leukemia. The possibility of plasma cell leukemia should always be considered in any patient who has leukemia in which the circulating cell has plasma-cell-like qualities, and appropriate diagnostic procedures for myeloma should be performed.