Although maternal haematopoiesis is characterised by rapid proliferation and immunological adjustment, leukaemia seldom occurs in pregnant women. In this case-control study, we investigated pregnancy and risk of acute myeloid leukaemia (AML). A total of 785 women with AML diagnosed in ages 15-50 were compared with 1576 age- and sex-matched controls. At the time of diagnosis, 13 cases and 53 controls were pregnant (1.3% and 3.4%, respectively), resulting in a significantly reduced odds ratio of 0.44 (95% confidence interval 0.22-0.85). Odds ratios of AML during the years following childbirth were close to unity. The results suggest that pregnancy conveys a strong short-term protection against AML.