A sample of 12,801 admissions to a pediatric hospital was surveyed in 1969-70 to determine the prevalence of disease which could be classified as "genetic" in origin or related to "congenital malformation"."Genetic" admissions accounted for 11.1% of the total while 18.5% were for congenital malformations; about 2% (unknown group) were probably genetic. Therefore about one third of all admissions represent the effect of abnormal gene-environment interrelations at some point in the development or life of the patient.The "genetic" patient is admitted more often to a medical service while the patient with congenital malformation usually goes to a surgical service; the former stays 7.3 days and the latter 8.6 days. A disproportionate number of patients staying longer than 10 days were found in the group with congenital malformations. Seventy percent of the patients with multiple admissions (3.2% of all admissions) have genetic illness or congenital malformation.