Aims: The aim of this study was to assess the influence of socioeconomic status on the risk of childhood leukaemia. Methods: A matched case-control design was used. The study population comprised all children (0 - 14 years old) born and reported to the Danish Cancer Registry between 1976 and 1991 for a diagnosis of leukaemia (n=377). Controls were selected from the Central Population Registry and matched by sex, age, and time of birth. Each child was assigned three categories of socioeconomic status, one corresponding to the annual average income in the municipality of residence at the time of birth, another corresponding to that at the time of diagnosis, and, finally, each family was assigned one of five social classes by use of the job titles of the parents. Conditional logistic regression was used to estimate the effect of socioeconomic status on the risk of childhood leukaemia. Results: Children born in low-income municipalities had a significantly increased risk of leukaemia (RR=2.71; 95% CI=1.41 - 5.21; p=0.003), which was higher among those who received their diagnosis before age five (RR=3.43; 95% CI=1.52 - 7.74; p=0.003). Neither individual social class nor the socioeconomic status of the residential area at the time of diagnosis was convincingly associated with the risk of childhood leukaemia. Conclusions: The results suggest that socioeconomic factors associated with community characteristics rather than individual lifestyle are related to the risk of childhood leukaemia and that these factors act early in life.