Two groups of children (aged 4-6 years) living in poor urban areas of Hermosillo (Sonora) and having "low" and "adequate" school achievements, were studied with the objective of determining their psychological, social, economic and nutritional characteristics. They were evaluated by means of standardized psychological tests. Their nutritional condition was assessed by dietary anthropometric and biochemical indicators. The socioeconomic characteristics of their micro-environment were also determined including the degree of home stimulation. It was found that school performance does not depend solely from their execution in psychological tests (Intelligence Quotient and visuomotor and linguistic performance) but also from other micro-environmental factors. The children with "low"school performance were characterized by sub-optimal energy intakes, mild-to-moderate undernutrition, belonged to large families, lived under more crowded conditions, and had a low level of home stimulation. It is concluded that poor nutrition and a disadvantageous micro-environment particularly home stimulation) are important factors related to the poor school performance of the children studied.