In spite of indisputable benefits, the use of antiretroviral therapy is associated with multiple metabolic complications. Switching to simpler regimens might maintain viral suippression, improve metabolic side effects, and provide insight into the pathogenesis of these complications. Our objective was to carefully characterize the virological and metabolic effects of switching from a successful protease inhibitor (PI)-based antiretroviral regimen to a nonnucleoside reverse transcriptase inhibitor (NNRTI)-based regimen with nevirapine (NVP). Forty patients, taking their first successful (less than 40 HIV RNA copies/ml) PI-based regimen, switched their PI to NVP. If patients did not tolerate NVP, substitution with efavirenz was allowed. The duration of the study was 48 weeks. At 12 weeks intervals subjects had multiple virological and metabolic parameters including glucose, insulin, C-peptide, glucagon, proinsulin, blood lipids, and lipoproteins. A subgroup of 18 patients also had body composition evaluations with DEXA scans and MRIs of the abdomen and the thighs as well as insulin tolerance tests. Ninety-five percent of the patients maintained viral suppression (95% CI 88-100%); only one patient failed and another developed hepatitis. There were improvements in glucose (decreased fasting glucose, insulin, and improved insulin tolerance) and lipid metabolism (decreased triglycerides and increased HDL), but no changes in body composition and bone mineral density. Our study supports a pathogenic role for PIs in the development of hypertriglyceridemia and insulin resistance, but a more limited role in the fat redistribution syndrome.