The present study, of microsocial nature, attempts to recover the biology-culture unity in the analysis of child undernutrition. A sample was selected of 52 children with primary undernutrition and their families, from the population attending the Nutritional Rehabilitation Unit of La Plata Children Hospital. The specific index was used to measure the low socioeconomic level in families with undernourished children. The mothers were interviewed to obtain information on the perception and recognition of undernutrition. Observation and interviews to the medical staff were performed in ten centers of primary health care (PHC), in order to reveal the medical practices related to this problem. The results obtained showed that the studied families conjugated the most unfavorable life conditions within poverty. Children with adequate weight at birth were nutritionally affected between 3 and 18 months of age, leading to height disorders. The representations and practices sustained by the mothers, relating undernutrition to a popular disease named "pata de cabra" lead to limitations in treatment. In PHC centers, conceptual and operative limitations are recorded for its detection and early treatment. It is concluded that cultural factors present in the two most directly related agents to child health (the mother and the PHC center) do not allow to modify the environmental effects that prematurely affect the children of poverty.