To test an instrument measuring the socioeconomic level, specially designed for low income urban populations, to be used in the identification of groups more vulnerable to infant malnutrition. This is a cross-sectional study, with a sample of 477 children aged 12 to 59 months old living in low income census sectors at "Vila Grande Cruzeiro", in Porto Alegre, RS. The children were submitted to anthropometric evaluation and their families were stratified according to socioeconomic level measured through an instrument developed in Chile for urban poor populations and adapted to local conditions. Children with weight/age, height/age and/or weight/height below - 2 standard deviation from the median of the reference population (NCHS) were considered malnourished. The children from families belonging to the lowest quartile in the socioeconomic classification had a 3.4 times higher chance of having low weight for age, a 2.7 times higher chance of being stunted and a 11.0 times higher chance of presenting low weight for height, controlling for potential confounders. The instrument measuring poverty level was useful to identify those children with higher risk for malnutrition.