The aim of this study is to clarify any correlation between nerve terminals (synapses) and proliferating extrinsic nerve fibers in the muscle layers of bowels affected by Hirschsprung's disease (HD). Synapses and extrinsic nerve fibers in the muscle layers of bowels of 10 patients with HD and 8 comparable controls were labeled with monoclonal antibody (MAb) 171B5 and acetylcholinesterase (AChE), respectively. In the control a rich and even distribution of synapses, representing neuromuscular junctions, was seen in the muscle layers, together with dense clusters of synapses in the adjacent myenteric plexuses; proliferation of extrinsic nerve fibers was not seen. In the transitional oligoganglionic segment of HD, many synapses were present in the myenteric plexus, but a few synapses in the muscle layers; there was a gradual increase of extrinsic nerve fibers from proximal to distal. In the narrow aganglionic segment very few synapses were seen in the muscle layers; proliferating nerve fibers and bundles were prominent. We conclude that the muscle layers of bowels affected by HD were almost denervated despite presence of intrinsic nervous elements in the oligoganglionic segment and proliferating extrinsic nerve fibers in the aganglionic segment.