Histochemistry to demonstrate cholinergic fibers in rectal suction biopsies is a reliable method to diagnose or exclude Hirschsprung's disease. The histochemical criterion of a positive reaction consists of the presence of many coarse discrete cholinergic fibers in the muscularis mucosae and in the immediately subjacent submucosa, best assessed under medium power with the condenser lowered. A marked increase in cholinergic fibers in the lamina propria is seen only in some cases of Hirschsprung's disease. At least some cases of Hirschsprung's disease may be the result of destruction of ganglion cells by a viral infection in the intrauterine and early postnatal life.