Abstract The effect of fat intake on total body (TB) and white blood cell (WBC) fat oxidation (FO) was studied during exercise in sedentary subjects. Four men and 6 women consumed diets of 19% and 50% energy from fat for 3 weeks each compared with their regular diet (30%). Before and after each diet, TB FO was measured at each stage of a treadmill test and WBC FO was determined. Fat intake had no effect on TB and WBC FO. Total body FO decreased as exercise intensity increased. WBC FO decreased postexercise as a result of an increase in WBC count. Total body FO in sedentary subjects was not influenced by diet at rest or during exercise. There was no effect from diet but exercise had a small effect on WBC FO. WBC FO may indicate a genetic predisposition for FO.