BACKGROUND: An increase in the protein content of a diet results in an increase in satiety and energy expenditure. It is not clear to what extent a specific type of protein has such effects. OBJECTIVE: The objective was to compare the effects of 2 diets with either 25% or 10% of energy from casein (25En% and 10En% casein diets), as the only protein source, on energy expenditure, substrate balance, and appetite profile. DESIGN: During a 36-h stay in a respiration chamber, 24 healthy subjects [12 men and 12 women, body mass index (in kg/m(2)) of 22.4 +/- 2.4, age 25 +/- 7 y] received isoenergetic diets according to subject-specific energy requirements: 25En% diet (25%, 20%, and 55% of energy as protein, fat, and carbohydrate, respectively) and 10En% diet (10%, 35%, and 55% of energy as protein, fat, and carbohydrate, respectively) in a randomized crossover design. Three days before the diets began, the subjects consumed a similar diet at home. Energy expenditure, substrate oxidation, and appetite scores were measured. RESULTS: The 25En% casein diet resulted in a 2.6% higher 24-h total energy expenditure (9.30 +/- 0.24 compared with 9.07 +/- 0.24 MJ/d; P < 0.01) and a higher sleeping metabolic rate (6.74 +/- 0.16 compared with 6.48 +/- 0.17 MJ/d; P < 0.001) than did the 10En% casein diet. With the 25En% casein diet, compared with the 10En% casein diet, the subjects were in positive protein balance (0.57 +/- 0.05 compared with -0.08 +/- 0.03 MJ/d; P < 0.0001) and negative fat balance (-0.83 +/- 0.14 compared with 0.11 +/- 0.17 MJ/d; P < 0.0001), whereas positive carbohydrate balances were not significantly different between diets. Satiety was 33% higher with the 25En% casein diet than with the 10En% casein diet (P < 0.05). CONCLUSION: A 25En% casein diet boosts energy expenditure, protein balance, satiety, and negative fat balance, which is beneficial to body weight management.