Between 1986 and 1991 773 infants were investigated by biopsy. 209 children suffered from a neuronal dysplasia of the submucous plexus (NID B). 64 of these 209 cases had concomitant Hirschsprung's disease with NID. The combination of Hirschsprung's disease with NID was established at biopsy not earlier than at 12 +/- 6 months of age. The classical form of an isolated aganglionosis had a median age at diagnosis of 4 +/- 2 months. The preconditions for a reliable diagnosis of NID are mucosal biopsies with submucosa taken 1, 3 and 9 cm above the pectinate line, the preparation of 15 microns thick serial sections, a acetylcholinesterase- and lactate-reaction and a systematic examination of all serial sections. Giant ganglia, which are 2-3 times as large as normal ganglia and having more than 7 LDH-positive nerve cells (10 +/- 3 nerve cells in the mean), are the most relevant parameters in the diagnosis of NID. They can be observed in infants as well as in adults. The NID proximal to aganglionosis is in principle not different from an isolated form of NID. Increase of acetylcholinesterase-activity in muscularis mucosae and lamina propria mucosae and a "hyperplasia" of the submucous plexus in early infancy disappears with advancing age and are very seldom observed at 2 years of age or in adulthood. NID B is the mildest form of a developmental abnormality of the autonomic nervous system, which shows in most cases a spontaneous normalization of gut motility.(ABSTRACT TRUNCATED AT 250 WORDS)