Although conferring a grave prognosis in patients with malignant lymphoma, high levels of serum lactic dehydrogenase (LDH) are usually not seen in patients with multiple myeloma, a more indolent tumor composed mainly of B cells in their terminal stage of differentiation. Thus, only 2 of 118 patients in earlier phases of myeloma showed marked LDH elevations to more than 500 U/L, whereas such abnormalities were present in 12 of 64 patients with advanced disease progressing despite treatment with vincristine, doxorubicin (Adriamycin), dexamethasone (VAD) (median LDH level, 700 U/L). High LDH levels were associated with high serum levels of beta-2-microglobulin, hypercalcemia, extraosseous disease features, a short preceding clinical course as well as a short subsequent survival time. A poor prognosis was also noted in patients with lower LDH in whom marked increments were induced by high-dose chemotherapy; thus, LDH elevations to greater than 300 U/L present before or found after high-dose cytotoxic therapy were observed in about 50% of patients with VAD-resistant myeloma and define a new clinical entity with features of extraosseous disease and an unusually aggressive course ("high-grade myeloma"). The shorter survival of newly diagnosed patients with high-normal compared with those with low-normal LDH levels ( < 200 U/L), regardless of tumor mass, suggests the presence in some patients of a tumor subpopulation with high LDH production that escapes growth control with standard treatment. Staging of multiple myeloma should therefore include measurements of serum LDH levels in addition to beta-2-microglobulin analysis and tumor mass estimation.