Accurate delineation of the intramural pathway of abnormal enteric nerve fibres in Hirschsprung's disease has previously proved impossible because the neural network is invariably transected in conventional histological sections. With the technique of wholemount immunohistochemistry (WI), the bowel segment is converted into a rectangular sheet and the serosa, long muscle (LM), circular muscle (CM), submucosa, and mucosa are separated into layers to allow each nerve plexus to be examined intact and neural pathways traced. The entire resected bowel specimens of nine HD infants and five infants serving as controls were investigated, using neuron-specific enolase and vasoactive intestinal peptide (VIP) for WI. The major new findings are (1) More VIP fibres were observed in aganglionic bowel with WI than with conventional sections; (2) Thick nerve trunks in aganglionic bowel do not descend from intrinsic neurons of oligoganglionic bowel as previously suggested, but have an extrinsic origin, accompanying blood vessels as small nerves initially, expanding subsequently, and ending blindly in submucosa; (3) CM nerve fibres follow muscle fibres concentrically for long distances in aganglionic bowel; and (4) LM nerve fibres meander in spirals in aganglionic bowel instead of running straight. This study shows that (1) WI is highly sensitive; (2) nerve fibres in aganglionic bowel have an extrinsic origin; and (3) innervation abnormalities in Hirschsprung's disease are not only quantitative but qualitative.