Studies have shown that the long chain fatty acid composition of a dietary fat influences whether it will be partitioned for either energy or storage. The objective of this study was to compare the effects of 3 oils differing in fatty acid composition on postprandial energy expenditure and macronutrient oxidation in healthy normal-weight men. Using a randomized crossover design, 15 subjects consumed breakfast meals containing 60% of energy as fat. The principal source of fat was (a) olive oil rich in oleic acid (18:1n-9), (b) sunflower oil rich in linoleic acid (18:2n-6), or (c) flaxseed oil rich in linolenic acid (18:3n-3). Measurements of resting metabolic rate, thermic effect of food, and postprandial energy expenditure were conducted with indirect calorimetry that recorded O(2) consumed and CO(2) produced one-half hour before meal consumption and 6 hours after meal consumption. Fat and carbohydrate oxidation rates were calculated from nonprotein gaseous exchange. Olive oil feeding showed a significant overall increase in energy expenditure compared with flaxseed oil (P < .0006) and a trend to increased energy expenditure compared with sunflower oil (P < .06). None of the 3 treatments exhibited significant effects on fat or carbohydrate oxidation. In conclusion, diets rich in oleic acid derived from olive oil may offer increased oxidation translating into increased energy expenditure postprandially.