The results of the treatment of acute lymphoblastic leukemia (ALL) in children depend not only on the biologic diversity of the leukemia cell, the multi-drug treatment schedule and the individual variability of drug metabolism, but also on the socioeconomic and cultural background of the leukemic child. Social and cultural disparity is very marked in underdeveloped countries and has been increasing in industrialized nations. The prognostic influences of these factors are poorly documented and sometimes mistakenly attributed to differences in ethnic origin. We have investigated in Brazil the relative impact of malnutrition and socioeconomic status on the outcome of ALL, adjusting for the known influence of biologic factors. Children with ALL (n = 167) treated with a Berlin-Frankfurt-Munster-based protocol were studied prospectively. At a median follow-up of 1623 days, the estimated probability of disease-free survival was 43 +/- 4%. The main cause for interruption of remission was bone-marrow relapse. Socioeconomic indicators of poverty (poor housing conditions, low per capita income and energy consumption) were significantly associated with a greater risk of relapse in univariate analysis. They were consolidated in a single index, socioeconomic status (SES), defined by the product of monthly per capita income times mean familial daily energy consumption. Other unfavorable findings included age, z score for the height for age at diagnosis (HAZ) below-1.28 and the z score for weight for age below-1.28. After adjustment in Cox's multivariate model, only HAZ and poor SES remained as predictive factors for relapse. Poor prognosis for leukemic children of low SES is just another indicator of social inequality.