The frequency of genetic disease among in-patients of Pediatric department, Seoul National University Hospital during two-year period from January 1972 to December 1973 was surveyed. A total of 1,413 admission records which included all but two-hundred sixteen patients whose records failed to specify primary diagnosis were subjected for present study. Primary diagnoses were classified into following seven categories: single-gene disorders, chromosomal abnormalities, polygenic disorders, probably genetic disorders, developmental anomalies, unknown etiology, and environmental group. The number of primary diagnoses and rate per 100 patients are as follows: Single-gene disorders, 25(1.8%); chromosomal abnormalities, 6(0.4%); polygenic disorders,127(9.0%); probably genetic disorders,43(3.0%); developmental anomalies, 22(1.6%); unknown etiology, 21(1.5%), and environmental group, 1,169(82.9%). Although environmental group is still major category of pediatric in-patients, the data obtained indicate the necessity for pediatricians caring the children of ability in an exact diagnosis and accurate family history taking, understanding of fundamentals of genetic principles as well as knowledge of recent literatures on specific disorders.