Despite numerous anecdotal reports of increases in weight if patients who begin aggressive new antiretroviral regimens, the magnitude and composition of weight gain has not been studied rigorously. Moreover, there are no data on potential mechanisms by which these increases in weight might occur. This study is designed to evaluate the effects of inhibition of viral replication on weight, body composition, resting energy expenditure, and energy intake in patients who are initiating potent antiretroviral therapy at the direction of their physician.