Obesity is a serious public health problem that effects 33 percent of the adult population and represents an accumulated economic cost of $56.3 billion annually. The purpose of this study was to assess the actual caloric intake and exercise caloric expenditure and the related changes in body composition following a low calorie diet with and without exercise. Fourteen obese males (average 43.8% fat) participated in a 10 week program to determine the effect of diet and exercise on body mass, fat mass (FM) and fat-free mass (FFM). Caloric intake and physical activity caloric expenditure were assessed to determine adherence to the prescribed intervention. The subjects were randomized into either a diet alone (D), or diet plus daily exercise (DE) treatment groups. The D group followed a 1200 Kcal/day diet while the DE group followed the same diet in addition to 60 minutes of moderate exercise each day. Actual calorie intake was analyzed from self reported daily food diaries using the Food Processor II software and exercise calorie expenditure was analyzed using a physical activity questionnaire (Stanford University). The average daily caloric consumption decreased 1238 calories per day from the beginning (3297 calories/day) to the end (2059 calories/day) of the study (P<.01) with no significant differences between groups. There was a significant change (P<.01) in calorie expenditure between the D and DE groups. The D alone group expended 520 calories/day initially with no change throughout the study. The DE group initially expended 490 calories/day and increased to 3411 calories at the end of the study (P<.01). The diet alone was associated with an average decrease in body mass of 9.5 kg (p<.01), all of which was fat mass (p<.001). The DE group lost an average of 14.2 kg of body mass (p<.001), of which 12.3 kg was fat mass (p<.001). Neither group has significant changes in FFM. Based on results of this study we conclude that: 1) the difference in caloric balance between the experimental groups was attributed to the exercise intervention, 2) a 1200 kcal/day diet for 10 weeks is sufficient to promote weight loss without a negative effect on fat-free mass and 3) the addition of daily aerobic exercise will promote further loss of body mass. Keyword(s): health promotion, physical activity, research