Benzene, an aromatic hydrocarbon that is a natural component of crude oil and natural gas, is toxic to the blood and blood-forming organs. Epidemiological studies have established an association between benzene exposure and acute myeloid leukemia, and increasing evidence also indicates a possible association between benzene and multiple myeloma. A specific benzene-associated myelodysplastic syndrome has also been suggested. Chronic hematotoxic effects of benzene exposure, including reduced lymphocyte, neutrophil and platelet counts in peripheral blood, have been detected at occupational exposure below a level that had previously been considered not to cause any health effects. Whether these abnormalities represent bone marrow damage and/or initial events in the development of a true neoplastic disease is not known. Together with a reported nonlinear relationship between benzene exposure and the level of various metabolites, favoring production of biologically reactive quinones at exposure below 1 part per million, these observations suggest that benzene even at low exposure levels may contribute to the risk of acute myeloid leukemia or myelodysplastic syndrome, especially among genetically susceptible individuals.