Objective: Nutritional status is assessed by measuring BMI or percent body fat (%fat). BMI can misclassify persons who carry more weight as fat-free mass and %fat can be misleading in cases of malnutrition or in disease states characterized by wasting of lean tissue. The fat-free mass index (FFMI) is proposed to assess body composition in individuals who have a similar body composition but differ in height allowing identification of those suffering from malnutrition, wasting or those that possess a relatively high muscle mass. The purpose was to determine whether the FFMI differs in a group of racially/ethnically diverse adults.Design:Cross-sectional.Subjects:Subjects were a multi-ethnic sample (Caucasian, CA; African American, AA; Hispanic, HIS and Asian, AS) of 1339 healthy males (n=480) and females (n=859) ranging in age from 18110 years. Total body fat, total fat-free mass and bone mineral density were estimated using dual energy X-ray absorptiometry. Results: FFMI differed among the four ethnic groups (P≥0.05) for both genders. A curvilinear relationship was found between age and FFMI for both genders although the coefficients in the quadratic model differed between genders (P≥0.001) indicating the rate of change in FFMI differed between genders. The estimated turning point where FFMI started to decline was in the mid 20s for male and mid 40s for female participants. An age × gender interaction was found such that the rate of decline was greater in male than female participants (P≥0.001). For both genders, FFMI was greatest in AA and the least in AS (P≥0.001). There was no significant interaction between race and age or age 2 (P≥0.06). However, male participants consistently had a greater FFMI than female participants (P≥0.001). Conclusions: These findings have clinical implications for identifying individuals who may not be recognized as being malnourished based on their BMI or %fat but whose fat-free mass corrected for height is relatively low. © 2011 Macmillan Publishers Limited All rights reserved.