The hospital records of 478 children with protein-calorie malnutrition (PCM) were reviewed. These represented all children diagnosed as malnourished during 1975 (3.6 per cent of all hospital admittances in the National Children’s Hospital); 52% of the cases were infants less than 6 months of age; 28% had low birth weight, a rate much in excess of the prevalence of low birth weight in the general population of Costa Rica (7%). In general, malnourished children had been weaned early, 75% during the first month of life. A considerable number of children belonged to "malnourishing families" which have particular characteristics favorable to establishment of malnutrition in the family. Thus, 36% of their siblings had also been admitted with malnutrition at a previous date to that of this study.