An increased release of free fatty acids (FFAs) into plasma likely contributes to the metabolic complications associated with obesity. However, the relationship between body fat and FFA metabolism is unclear because of conflicting results from different studies. The goal of our study was to determine the inter-relationships between body fat, sex, and plasma FFA kinetics. We determined FFA rate of appearance (R_a) in plasma, by using stable isotopically labeled tracer techniques, during basal conditions in 106 lean, overweight, and obese, nondiabetic subjects (43 men and 63 women who had 7.0-56.0% body fat). Correlation analyses demonstrated: (i) no differences between men and women in the relationship between fat mass (FM) and total FFA R_a (micromol/min); (ii) total FFA R_a increased linearly with increasing FM (r = 0.652, P < 0.001); (iii) FFA R_a per kg FM decreased in a curvilinear fashion with increasing FM (r = -0.806; P < 0.001); (iv) FFA R_a in relationship to fat-free mass (FFM) was greater in obese than lean subjects and greater in women than in men; and (v) abdominal fat itself was not an important determinant of total FFA R_a. We conclude that total body fat, not regional fat distribution or sex, is an important modulator of the rate of FFA release into plasma. Although increased adiposity is associated with a decrease in fatty acid release in relationship to FM, this downregulation is unable to completely compensate for the increase in FM, so total FFA R_a and FFA R_a with respect to FFM are greater in women than in men and in obese than in lean subjects.