Benzene is known as a severe bone marrow poison. In the last 40 years, however, more than 100 cases of leukemia attributed to benzene have been described besides cases of classical pancytopenia. It is possible that many cases of acute leukemia developing as a terminal stage of an aplastic anemia due to benzene have been missed in the past, when bone marrow puncture was not performed. Benzene leukemia is mostly an acute stem cell or myeloblastic leukemia, sometimes aleukemic; in some cases it has shown a pattern of an erythroleukemia. The Institutes of Occupational Health of Milano and Pavis have collected 25 cases of benzene leukemia, all of them acute. Chronic types of leukemia are also said to occur after exposure to benzene. There may be a latent period extending over several years between cessation of exposure to benzene with more or less pronounced anemia, and the onset of leukemia. Benzene is known to be a mutagenic agent inducing chromosomal aberrations, and this may lead to the development of an abnormal cell clones with selective growth advantage. It is also possible that benzene may activate a latent leukemogenic virus or produce an immunological impairment with proliferation of abnormal cell clones as a result of defective immune surveillance.