Anatomically, diaphragmatic agenesis (DA) is the most extreme form of congenital diaphragmatic defect, but clinically it has not been defined separately from Bochdalek's hernia (BH). Between 1986 and 1992, the authors treated 55 neonates who had diaphragmatic defects. Forty-eight of these cases presented within 24 hours of birth. Seventeen of these neonates (35.4%) were found to have DA; the other 31 (64.6%) had BH. There were no significant differences in maternal age, gestational age, gender ratio, birth weight, and incidence of coexisting congenital anomalies between the two groups. However, neonates with DA differed significantly from those with BH with respect to incidence of antenatal diagnosis (76.4% v 12.5%, P = .0004), mean Apgar scores at 1 (4.1 +/- 2.0 v 5.7 +/- 2.3; P = .034) and 5 (5.5 +/- 2.7 v 7.6 +/- 2.2; P = .016) minutes, mean duration of preoperative stabilization (2.8 +/- 2.0 v 2.1 +/- 1.9 days; P = .044), and postoperative respiratory support (27.7 +/- 13.6 v 9.3 +/- 8.0 days; P = .002). Complications occurred in all seven DA survivors (100%) and in only four (19.0%) of the 21 BH survivors (P = .0008). The long-term survival rate was significantly lower for neonates with DA (29.4% v 64.5%; P = .04). Diaphragmatic agenesis is a distinct clinical entity; its unique short- and long-term problems require careful management.