Anatomically, diaphragmatic agenesis (DA) represents the most extreme form of congenital diaphragmatic defects, but clinically it has not been defined separately from Bochalek's hernia (BH). Between 1986 and 1992, the authors treated 55 neonates who had diaphragmatic defects. Forty-eight of them presented within 24 hours of birth and comprised the study group. Seventeen neonates (35.4%) were found to have DA; the other 31 (64.6%) had BH. There were no differences in the maternal age, gestation course, gender ratio, birth weight, or incidence of co-existing congenital anomalies between the two groups. However, there were significant differences with respect to the incidence of antenatal diagnosis (76.4% for DA patients v 12.5% for BH patients; P = .0004), mean (+/- SD) Apgar scores at 1 (4.1 +/- 2.0 v 5.7 +/- 2.3; P = .034) and 5 minutes (5.5 +/- 2.7 v 7.6 +/- 2.2; P = .016), mean duration of preoperative stabilization (2.8 +/- 2.0 v 2.1 +/- 1.9 days; P = .044) and mean duration of postoperative respiratory support (27.7 +/- 13.6 v 9.3 +/- 8.0 days; P = .002). Complications occurred in all seven survivors of DA and in only four (19.0%) of 21 survivors of BH (P = .0008). The neonates with DA had a significantly poorer long-term survival rates (29.4% v 64.5%; P = .04). Diaphragmatic agenesis is a distinct clinical entity; its unique short-term and long-term problems require careful management.