: Currently available protease inhibitors are associated with development of a group of metabolic disorders. These include a peripheral lipodystrophy syndrome in which there is fat wasting in the face, arms, and legs; fat accumulation in the abdomen, dorsocervical region, and/or breasts (women only); as well as hyperlipidemia, hypercholesterolemia, and insulin resistance. A review of 15 observational studies and case reports shows that the incidence of the peripheral lipodystrophy syndrome increases with time of exposure to protease inhibitors, with a> 60% incidence seen after 1 year of continuous treatment. Protease inhibitors are hypothesized to cause this syndrome by impairing conversion of retinoic acid to cis-9-retinoic acid (leading to impaired peripheral fat storage, sequestration of body fat to central adipocytes, and hyperlipidemia) and by inhibiting low-density lipoprotein receptor-related protein (LRP), thus preventing postprandial chylomicron clearance and further contributing to hyperlipidemia. Recent in vitro data suggest that more than one pathway contributes to the lipodystrophy syndrome and that pathways may differ among protease inhibitors. Although the central fat accumulation, hyperlipidemia, and insulin resistance components of this syndrome may reverse after discontinuation of protease inhibitor therapy, it is not known whether complete normalization of fast-wasted body regions is possible. Prospective controlled studies are needed to define whether protease inhibitors currently under development are less prone to produce the lipodystrophy syndrome.