Neuronal intestinal dysplasia (NID) clinically resembles Hirschsprung's disease but is characterized by hyperplasia rather than aganglionosis of the intramural plexus. Surgical intervention is common. We report the 5-year follow-up of an infant with the mixed form of NID managed medically and a method by which NID can be quantified histologically. Hyperganglionosis was determined by counting the number of ganglia per high-power field and the number of ganglion cells per ganglia from at least two biopsy specimens. The patient's biopsies and biopsies from "normal" and "inflamed" patients were compared. Normals contained 0.68 +/- 0.28 (mean +/- SD) ganglia per high-power field and 2.16 +/- 0.31 ganglion cells per ganglion. The inflamed biopsies were similar, 0.69 +/- 0.38 ganglia per high-power field and 2.63 +/- 0.40 ganglion cells per ganglion. The patient's initial rectal biopsy revealed 7.6 ganglia per high-power field and 3.8 ganglion cells per ganglion. Management of the patient included saline colonic irrigations and hyperalimentation with gradual reinstitution of breast-feeding. Clinical improvement was associated with normalization of manometry and biopsy findings, a phenomenon not documented previously in the literature. Irrigations were stopped at age 9 months, and the child is now asymptomatic.