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We studied the epidemiology of diaphragmatic hernia based on 1439 cases collected from a French, a Swedish, and a California birth defect registry. This is the largest epidemiological material available up to now. Isolated diaphragmatic hernia existed in 875 infants, diaphragmatic hernia with associated malformations in 486, and with chromosome anomalies in 78. Among unilateral forms, right-sided hernias were found in about 20%, equally often in isolated and associated forms. There is a male excess of the same magnitude in isolated and associated forms but among the latter varying between programs. There was a change in the sex distribution of associated cases in California before 1988 and after which is not fully explained. The twinning rate is increased and is similar in isolated and associated forms. There is no marked effect of maternal age or parity on the risk of diaphragmatic hernia. Except for well-known conditions, no specific combination of associated malformations was found. Marked differences in recorded rates were found between the programs but not between races within the California program. The differences appear to a large extent to be due to different ascertainment, also apparent in different survival rates. Marked variations in rate with time is seen in the two programs with an extended observation time, again at least partly explainable by varying ascertainment.