The outcome of two populations of twins delivered at the same hospital, numbering 554 in 1963 to 1972 and 614 in 1978 to 1984, was reviewed to determine the factors contributing to depression at birth, trauma, and mortality in each period. The cesarean section rate had increased from 3% in the early period to 51% in the later period, with 92% of the later cases in which the first twin presented abnormally being delivered by cesarean section. Among infants of greater than 28 weeks' gestation the incidence of severe depression at birth was not reduced with the increased cesarean rate, remaining at 2% in both populations; none developed encephalopathy or died as a result of birth asphyxia or trauma. Neonatal mortality was markedly reduced in the second period, primarily because of a reduction in deaths resulting from respiratory distress syndrome. It is not possible to show that the marked increase in the rate of cesarean delivery has improved the condition of twin infants at birth.