The cases of 23 patients whose condition was diagnosed as truncus arteriosus, type I or II, and who were seen at the Mayo Clinic during the decade preceding 1967, that is, before corrective operation became feasible, were reviewed. Ten were infants (through one year of age), and all ten have died. Eight ranged in age from more than one year through seven years of age, and all are living, except one, who diet 11 years after diagnosis. Five were older than seven years, and all had severe pulmonary vascular obstructive disease; three have died. Thus, 14 of the 23 have died, and all but one surviving patient have mild to moderate disability. The generally grave prognosis for patients with truncus arteriosus warrants continued use of corrective operation, but suggests that the greatest benefit can be realized by successful correction in the infant with congestive heart failure and in early childhood before the development of severe pulmonary vascular obstructive disease.