We describe an infant with Hirschsprung disease (congenital aganglionosis of the intestine) involving the colon and terminal ileum. Midtrimester prenatal diagnosis of this disorder in this infant was attempted utilizing amniotic fluid disaccharidase analyses, ultrasound, and amniography. Decreased disaccharidase activities in amniotic fluid have been reported previously in association with other forms of intestinal obstruction. At 15 weeks' gestation, normal amniotic fluid disaccharidase levels were obtained. Serial ultrasound evaluations did not indicate any pathology, and the results from amniography were inconclusive. The implication of the normal disaccharidase values is that Hirschsprung disease may in some cases result from degeneration of intestinal ganglia after 16 weeks' gestation rather than from faulty migration of neural crest cells. The inheritance of Hirschsprung disease is generally consistent with sex-modified multifactorial inheritance with a lower threshold of expression in males. The case we report has a family history of three affected first- and second-degree relatives. Autosomal dominance with variable expressivity is a possible explanation in this family.