We assessed the familial occurrence of Hirschsprung's disease from 224 probands born in Denmark after 1959. Probands who were still alive received a mailed questionnaire, and medical reports for the probands and their relatives with suspected Hirschsprung's disease were examined. The diagnosis of Hirschsprung's disease required a histologically verified biopsy or surgical colonic specimens, and exclusion of a secondary causes for Hirschsprung's disease. Familial occurrence was seen in 11 families. Ten first-degree, two third-degree and one fifth-degree relatives had Hirschsprung's disease. Both short segment agangliosis (the sigmoid colon or below) and long segment agangliosis (above the sigmoid colon) occurred in five of the 11 families, implying that the etiology of Hirschsprung's disease with short and long segment agangliosis is the same. Compared with the general population, the first-degree relatives of the 224 probands had a minimum of a 93-fold increased risk of Hirschsprung's disease. This strongly suggests that genetic factors play a role in Hirschsprung's disease.