PURPOSE: Age can affect the delicate physiologic balance of the internal anal sphincter diameters and pressure governed by Laplace's law. This study compares the effect of aging on the internal anal sphincter thickness and diameter in younger and older nulliparous females without symptoms of fecal incontinence undisturbed by an endoanal probe. METHODS: Magnetic resonance images were selected from a large database of nulliparous females to form two groups: "younger" females, aged 30 years and younger (n = 32), and "older" females, aged 50 years and older (n = 32). All patients were scanned without endoanal coils to allow undistorted measurement of the internal anal sphincter diameters. Inner and outer diameters were measured from axial magnetic resonance images and used to calculate sphincter thickness and mean radius by two independent investigators blinded to patient age. RESULTS: The mean age in the younger group was 26 +/- 2.8 years, whereas that of the older group was 61.8 +/- 7.6 years. Older females had a 33 percent thicker internal anal sphincter (younger vs. older: 4.5 +/- 0.7 vs. 5.9 +/- 1 mm; P < 0.001), a 20 percent larger inner diameter (7.1 +/- 1.3 vs. 8.5 +/- 1.8 mm; P = 0.001), and a 27 percent larger outer diameter (16 +/- 2.1 vs. 20.3 +/- 3.3 mm; P < 0.001) than younger females. Neither sphincter thickness nor inner or outer diameter correlated with body mass index. CONCLUSIONS: There is an increase in internal anal sphincter thickness, inner diameter, and outer diameter, which correlates with age in asymptomatic nulliparous females.