BACKGROUND & AIM: Regular physical activity prevents or limits weight gain, and gain in body mass index (BMI) and decreases mortality. The aims of the study in healthy adults were to determine the differences in fat-free mass index (FFMI) (kg/m(2)) and body fat mass index (BFMI) between age groups and determine the association between physical activity and FFMI and BFMI. METHODS: Caucasian men (n=3549) and women (n=3184) between 18 and 98 years, were classified as either sedentary or physically active (at least 3h per week at moderate or high-intensity level activity). FFMI and BFMI were measured by 50 kHz bioelectrical impedance analysis.RESULTS: BFMI was significantly higher (P<0.05) in sedentary than physically active subjects and the differences became progressively greater with age. The physically active subjects were significantly less likely to have a low or high FFMI (logistic regression, P<0.001), and a high or very high BFMI (P<0.001), and more likely to have low BFMI (P<0.001) compared to sedentary adults. In contrast with fat-free mass, which was lower in older subjects, the height-normalized FFMI was stable with age until 74 years and lower thereafter. Significantly higher BFMIs were noted in older subjects.CONCLUSION: Physically active subjects are less likely to have low or high FFMI, and high or very high BFMI, and more likely to have low BFMI. In contrast to common claim that fat-free mass decreases with age, we found that FFMI was stable until 74 years. The use of FFMI and BFMI permits comparison of subjects with different heights and age.