Serum total lactic dehydrogenase (LDH) levels were examined in 42 patients with acute leukemia, 9 patients with chronic myeloid leukemia, 6 of them in blastic crisis, and 53 patients with lymphoma and other lymphoproliferative disorders. The mean range of serum LDH levels in Hodgkin's and non-Hodgkin's lymphoma was 402 ± 210 IU/liter and 313 ± 113 IU/liter, while that of patients with nonmalignant disorders was 308 ± 74 IU/liter. In acute nonlymphoblastic leukemia (ANLL), the range was 126-684 IU/liter (mean value of 413 ± 146 IU/liter). In 6 of the patients (11.3%) with lymphoma and in 6 cases (26.8%) with ANLL, the LDH levels were above 500 IU/liter. None of these patients had levels over 900 IU/liter. Patients with acute lymphoblastic leukemia (ALL) had a range of 402-3582 IU/liter (mean value of 1669 + 1038 IU/liter). In 15 of the 19 patients (78.9%) with ALL, serum LDH values were above 900 IU/liter. In addition, 3 patients with chronic myeloid leukemia (CML) in blastic crisis had levels of 970-1940 IU/liter. One of these 3 patients had lymphoblastic crisis, while the second case responded clinically to vincristine and prednisone, but was not regarded as ALL. The differences in serum LDH levels between ALL and ANLL are statistically significant (p < 0.001). It appears that markedly elevated serum LDH levels in acute leukemia are suggestive of ALL, and that in individual patients, the LDH levels were correlated with the number of blasts during remission and relapse.