BACKGROUND: Cross-sectional data have shown that sarcopenia and fat accumulation are associated with aging and can be limited by structured physical training. However, it is often difficult to maintain a long-term compliance to training programs. It is not clear whether leisure-time physical activity is effective in preventing sarcopenia and fat accumulation. OBJECTIVES: (i) To investigate longitudinal body composition changes in a population of elderly people in good apparent health. (ii) To evaluate the impact of leisure-time physical activity on muscle mass and characteristic as reflected by total body potassium per fat-free soft tissue (TBK/FFST), and on fat accumulation. DESIGN: Longitudinal evaluation over 3 years, of body composition changes in 74 healthy men and 66 women, over 65 years old. Body fat and FFST were analyzed by dual-energy X-ray absorptiometry and TBK by whole-body (40)K counter. Physical activity was analyzed by a specific questionnaire. RESULTS: Despite a stable total body weight, FFST and appendicular skeletal muscle mass slightly decreased (-0.3+/-1.4 and -0.2+/-2.2 kg, P<0.01, respectively) as well as the TBK/FFST (-4.1+/-6.3 mmol/kg, P<0.001), over the 3-year period. Body fat increased significantly (0.6+/-2.2 kg, P<0.0001), and it accumulated mainly in the abdomen (0.4+/-1.5 kg, P<0.01). Multiple regression analysis showed that body composition changes were related mainly to body weight changes. Nevertheless, positive linear correlations were observed between the degree of engagement in leisure-time physical activity and FFST (P<0.01), appendicular skeletal muscle mass (P<0.05), TBK/FFST (P<0.05), whereas negative correlation was observed with total and truncal fat (P<0.01). CONCLUSIONS: Mild but significant decline in muscle mass and its TBK content, and body fat accumulation were observed over a 3-year period in healthy elderly subject: leisure-time physical activity does not seem to prevent them. However, a higher level of physical activity is associated with higher muscle mass and TBK content, and less total and truncal fat.