A survey of 2,945 admissions to the National Institute of Pediatrics, México, was undertaken to document the frequency and burden of genetic disorders. Patients were classified into five categories: single gene disorders; chromosomal disorders; complex genetic etiology; unknown etiology, and nongenetic disorders. Of all admissions studied, 4.3% had a genetic disease (categories 1 and 2); 33.5% had a partly genetic condition (category 3); 13.8% had a disorder of unknown etiology; and 48.4% had an environmental disease. In terms of economic burden, patients with genetic or partly genetic disorders came more frequently from outside the city and had more and longer admissions with a higher number of surgeries. The results suggest that genetic disease, although less frequent than disease caused by environmental conditions, lays a considerable financial burden on the hospital and on the affected families. The need to create genetic units in developing countries is emphasized.