Frequencies of 35 HLA A, B, C, and DR antigens were determined in 1,834 leukemic Caucasoids to evaluate possible associations between HLA and leukemia. In comparison with the frequencies of HLA antigens in published controls, the frequency of Cw3 was significantly higher in patients with acute lymphoblastic leukemia (relative risk = 2.64, P less than 0.0002), acute myelogenous leukemia (relative risk = 1.92, P less than 0.0007), and chronic myelogenous leukemia (relative risk = 2.07, P less than 0.002; P values adjusted for multiple comparisons). The frequency of Cw4 was elevated in patients with acute lymphoblastic leukemia (relative risk = 2.01, P less than 0.0003), acute myelogenous leukemia (relative risk = 2.06, P less than 0.0002), and chronic myelogenous leukemia (relative risk = 2.14, P less than 0.0008). The frequency of Aw19 was significantly decreased in patients with acute myelogenous leukemia (relative risk = 0.68, P less than 0.01) and chronic myelogenous leukemia (relative risk = 0.59, P less than 0.005). None of the other 32 HLA antigens investigated had a statistically significant association with leukemia. The data suggest that Cw3 and Cw4 may be markers for leukemia susceptibility genes, while Aw19 may be a marker for decreased susceptibility to leukemia.