This study aimed to investigate the influences of age and sex on the thickness of the rectus abdominis muscle and subcutaneous fat at the abdomen. The two tissue thickness values were determined using ultrasonography in a total of 194 young (20-29 years) and elderly (70-79 years) persons of both genders. In both sexes, the elderly groups showed significantly thinner muscle and thicker fat than the younger groups. The percentage of the mean value of subcutaneous fat thickness in the elderly group compared to that in the younger group was higher in the women (176%) than in the men (128%), but the corresponding value of muscle thickness was similar between men (68%) and women (65%). In both generations, the women had significantly thinner muscle and thicker fat than the men. The percentage of the mean value of subcutaneous fat thickness in the women compared to that in the men was higher in the elderly group (169%) than in the younger group (123%), but the corresponding value of muscle thickness was similar between the younger (80%) and elderly (77%) groups. In every group, subcutaneous fat thickness significantly correlated with waist circumference, but muscle thickness did not. The findings here indicated that: (1). aging was associated with a decrease of muscle thickness and an increase of subcutaneous fat at the abdomen in men and women, with a similar relative loss of muscle thickness in both genders and a greater increase of subcutaneous fat thickness in women than in men, and (2). subcutaneous fat rather than muscle thickness reflected waist circumference, regardless of age and gender.