The incidence of Hirschsprung's disease was studied in approximately 1.5 million consecutive live births in Denmark by hospital records. A diagnosis of Hirschsprung's disease required a histologic verified absence of ganglion cells in either biopsy or surgical colonic specimens. The incidence of Hirschsprung's disease was found to be 0.140 per 1000 live births (1:7,165) with a male: female ratio of 4.1:1 in short segment, and 2.4:1 in long segment Hirschsprung's disease (p = 0.36). Maternal age and birth order were unimportant factors. The association of Hirschsprung's disease and Down's syndrome was seen in 9 of the 207 patients and may represent a real association, whereas the association with congenital heart defects seen in 2% (not including patients with Down's syndrome) is more doubtful. A mortality of 16% among the patients with Hirschsprung's disease emphasizes the extreme importance of early diagnosis.