ISEE-373 Objective: Benzene is a well-known human carcinogen (IARC group 1; leukemia and possibly lymphoma). It is emitted by a large Swedish refinery (start of production 1975), which also emits olefins (cracking products: ethylene, 1,3-butadiene, and propene, suspected human carcinogens, IARC 2A-B). The aim of this study was to investigate whether the occurrence of leukemia was increased in the general population living close to the refinery. Material and Methods: In an ecologic study, the incidence of leukemia was investigated in the 2 nearby downwind parishes and compared with the expected incidence using age-, sex-, and calendar-specific rates in the region of West Sweden (1.5 million people). This was done also for 5 other nearby parishes, similar with respect to geology, population, and hospital care, but not located downwind the refinery. Results: The SMR for leukemia was increased in 1975 to 2004 (33 cases vs. 22 expected; SMR 1.5, 95% CI 1.01–2.1). The increase was concentrated to the period 1995 to 2004 (19 cases vs. 8.5 expected; SMR 2.2, 95% CI 1.4–3.5). In the other 5 parishes, findings were normal (1975–2004: 50 cases vs. 56 expected; 1995–2004: 17 cases vs. 21 expected). Total cancer was as expected in all parishes. The contributions from refinery emissions to ambient levels were, however, estimated to be low (benzene up to 2 μg/m3 in the 2000s). Calculations of the theoretically expected excess risk, using the US EPA extrapolations from the Pliofilm cohort, and unit risks for the olefins, resulted in an expected excess risk of ≪1 case in a 10-year period. Conclusions: The clear increase of leukemia contrasts with the negligible risk expected from generally accepted dose-response data, for unknown reasons. There were no cluster/index cases. Parishes and time periods were also selected a priori. Chance, incorrect dose-response assumptions at low-level exposure to benzene and olefins, or unknown environmental, diagnostic, or hereditary factors are possible explanations.