Seven obese women (greater than 30 per cent body fat, mean = 43 per cent) performed moderate exercise (30 to 40 per cent of maximal oxygen consumption) for 90 min during head-out immersion in cool water (17 to 22 degrees C) five times per week for eight weeks to determine if cold exposure and the attendant caloric deficit in body heat stores would lead to body weight loss. Exercise was on a cycle ergometer that was modified for underwater use. Body weight, body fat, fat-free body weight, and caloric intake did not change significantly as a result of the exercise program in cool water. The significant heat debts incurred were presumably repaid without an increase in energy expenditure. The caloric balance and the thermal data obtained suggest that body heat stores were slowly repaid during recovery by resting metabolic heat production associated with a reduced rate of body heat loss. Maximal oxygen uptake did not increase significantly. No metabolic or thermal evidence was found to support cold adaptation in the obese women. Ergometer exercise in water as well-tolerated, as evidenced by no exercise-related injuries and an adherence rate of 93 per cent. It was concluded that, while cold exposure does not increase caloric expenditure significantly in obese individuals, exercising regularly in cool water may be beneficial as it may motivate obese people to exercise at higher intensity for thermal comfort and the water environment may help prevent injuries.