Studies of risk factors for acute leukaemia are inconclusive. This case–control study was done in West Azerbaijan province, Islamic Republic of Iran, to determine the relationship between birth weight and acute leukaemia in children aged under 15 years. For every patient 2 age- and sex-matched controls were selected from hospital and community populations. Of 130 cases diagnosed over the period 2003–2009, 108 (83.1%) had lymphoblastic and 22 (16.9%) myloblastic type. Significantly more of them were male than female (55.4% versus 44.6%). In a multivariate logistic regression model variables significantly associated with acute leukaemia were birth weight (OR = 2.25), birth order (OR = 2.25), birth place (OR = 7.93), history of chickenpox (OR = 0.46) and mothers’ education (OR = 3.23). The risk of acute leukaemia increased significantly with increasing birth weight in the total group and among girls, but not among boys.