In order to determine the possible implication of elastin in spasticity of the aganglionic segment in Hirschsprung's disease the elastic fibers in the colon at rectosigmoid level were studied in seven surgical specimens of aganglionic bowel and in seven normal controls. Elastic fibers in both the muscle layers of normal bowel are thin, tend to be straight, and follow the line of muscle fasciculi. In aganglionic bowel, however, the fibers are more numerous and thicker in both layers, and in the longitudinal layer they are laid down in spirals. The total elastin content is increased by approximately 100% as compared with controls. These structural and quantitative changes in the elastin may contribute both to the spasticity and to the increased elasticity of the aganglionic segment.