Low carbohydrate diets, including high-fat, ‘ketogenic’ diets, have moved to the forefront of popular diets promoted for weight loss in the United States. These diet plans have been aggressively marketed to millions of Americans every year. However, there has been virtually no rigorous study of low carbohydrate, high fat diets either with regard to efficacy in promoting weight loss or effects on cardiovascular risk factors. Because very low carbohydrate diets (i.e., the ketogenic diets) recommend increased intake of fat, and saturated fat, we hypothesized that following one of these diet plans could actually worsen important risk factors for cardiovascular disease. This is an issue of great importance because large numbers of people experiment with low carbohydrate diets despite not knowing their effects on blood pressure, plasma lipid levels, and glucose tolerance. Therefore, we have proposed a clinical trial to test our hypothesis. In this study, we will compare the effects of three months of a ketogenic diet (as promoted by Dr. Robert Atkins) with three months of a weight loss diet that conforms to the American Heart Association dietary guidelines in a group of 100 otherwise healthy obese subjects. During the course of the study, we will determine the effects of diet on 1) body weight, percent fat, and resting metabolic rate, 2) blood pressure, 3) plasma lipids, 4) measures of glucose tolerance, and 5) measures of psychological health and eating behaviors. This pilot study will provide the preliminary data needed to demonstrate the efficacy (or lack thereof) of a popular weight loss technique (i.e., the ketogenic diet) that is currently being criticized by professional organizations. Armed with preliminary data, the investigators plan to further investigate the long-term anthropometric and metabolic effects of ketogenic diets with a larger sample.