The aim of this study was to investigate seasonal trends in the incidence of acute lymphoblastic leukaemia (ALL) around the times of birth and diagnosis in children aged 0-4 years and also to examine gender specific effects. Children born in South Hungary during 1981-1997 were analysed. Registrations of first malignancies for children, diagnosed under age 5 years before the end of 2002 were obtained from the Hungarian Paediatric Oncology Group providing a representative sample of Hungarian children over a 17 year period of time. Data were available on the corresponding numbers of births for each month of the study period were obtained. Statistical analyses were performed using logistic regression with harmonic components. The study analysed 121 cases of children, aged under 5 years, who were diagnosed with ALL. We found no seasonal effect related to date of diagnosis. However, there was seasonal variability for ALL related to date of birth. Maximal rates were seen in children born in February and August in the simple harmonic regression model for all children diagnosed with ALL. Analysis by gender found evidence of seasonality related to month of birth with peaks in February and August in boys, but different seasonal effects were seen for girls (peak in November, nadir in May). Our study provides some evidence that male specific immune responses to infections around the time of birth could explain the male predominance in the incidence of ALL.