A population-based case-control interview study of 309 childhood leukaemia cases and 618 age and sex matched controls showed a significant dose-response relation between chloramphenicol and risk of both acute lymphocytic leukaemia (ALL) and acute non-lymphocytic leukaemia (ANLL), treatment for more than 10 days being associated with risks of 11.0 and 12.0, respectively. A significant risk of ANLL was also observed with the use of syntomycin, a drug pharmacologically related to chloramphenicol. The risks remained high when analyses were limited to either first or latest use of these antibiotics more than 2 years before diagnosis. Although the association may have non-causal explanations, the results warrant cautious prescribing patterns and further investigations into the leukaemogenic potential of chloramphenicol.