Anal endosonography provides clear images of the internal anal sphincter. Forty-two controls have been studied to establish a range for thickness and echogenicity of the normal internal sphincter, and any physical correlate for these observations. No relationship was found between the thickness of the internal anal sphincter and body weight, height or gender, but there was a significant correlation for thickness with age (p less than 0.001), the 95% confidence interval being 2.4-2.7 mm less than 55 years and 2.8-3.4 mm greater than 55 years. Hyperechogenicity of the internal sphincter was significantly associated with an age greater than 55 years (p less than 0.01) and a thickness greater than 2.8 mm (p less than 0.05). This has not been observed previously and suggests a histological change in the sphincter as it ages.