To investigate the association of childhood ALL and SES, we collected data on 507 children aged 0-14 years diagnosed with ALL between 1997 and 2002 from the population-based Cancer Registry of São Paulo, Brazil, covering 96 districts. Each child was assigned to an SES category based on the district of residence at diagnosis. Four SES categories were created from high to low, based on the Social Exclusion Index (SEI) that includes the following indicators: poverty, employment, inequality, education and violence. Using Census data, cases were also classified into quintiles according to the percentage of households with more than 7 persons. Age-specific and age-standardized incidence rates were calculated using the 1960 world standard population. Age-adjusted incidence rate was 3.68/100,000 for males and 2.87/100,000 for females. Children living in areas with lowest SES presented a significant lower risk of ALL compared with those living in the wealthiest districts (Rate Ratio [RR] = 0.34; 95% Confidence Interval [CI] = 0.28-0.44). Lower incidence rates of childhood ALL were also found in areas with high percentages of households with more than 7 persons (>or=5.7%) compared with areas where there were <or=2.2% (RR = 0.32, 95% CI 0.25-0.43). There was a strong correlation between SEI and crowding (rho = -0.95, p < 0.001). Population-based attributes for SES and household size may be useful surrogate markers of early exposure to childhood infections, which has been found to decrease the risk of ALL.