One quarter to one third of all children born alive with congenital heart disease are dead by the end of the first month of life. To decrease the overall mortality of congenital heart disease, early diagnosis and management in this age group are important. One hundred infants under 1 month of age underwent cardiac catheterization during a 4½-year period (1964-1968). The indications for the investigation were congestive cardiac failure alone in 45%, cyanosis alone in 23%, and a combination of heart failure and cyanosis in 30%. A diagnostic classification of the cases is presented and the major groups are briefly outlined. The total mortality was 57%. Fifty-three patients underwent cardiac surgery and 22 (42%) survived. Nineteen patients died within 24 hours of cardiac catheterization without operation. The high mortality after surgery and cardiac catheterization in this age group is emphasized, and the factors involved are discussed.