Many cases of congenital diaphragmatic hernia (CDH) are currently detected before birth. The authors hypothesized that there is a subgroup of patients with CDH who have no evidence of visceral herniation in utero and who would be expected to have less pulmonary hypoplasia and a good prognosis. Among 41 neonates with left-sided CDH treated between January 1990 and October 1993, 17 cases were diagnosed after birth. Ten of the 17 had undergone detailed fetal sonographic imaging at or after 20 weeks' gestation. After independent review of the prenatal scans of these 10 patients, one was found to have evidence of a diaphragmatic hernia and was excluded from further analysis. The other nine fetuses survived, and prosthetic repair of the diaphragmatic defect and extracorporeal membrane oxygenation (ECMO) were each required in only one patient. This contrasted with the outcome for 18 control patients with prenatally diagnosed CDH: 4 (22%) died, 13 (72%) required prosthetic repair, 9 (50%) were supported with ECMO and the duration of ventilatory support and hospital stay were significantly longer. There is a spectrum of severity among both pre and postnatally diagnosed cases of CDH. In the neonate with an isolated left-sided diaphragmatic hernia, a good prognosis is to be expected if the condition was not detectable by detailed prenatal sonography in the second half of pregnancy.