The metabolic syndrome consists of a cluster of abnormalities that include hypertension, abdominal obesity, impaired fasting glucose, elevated fasting triglyceride levels, and low HDL-cholesterol (HDL-C). Patients with metabolic syndrome are at increased risk of cardiovascular disease and, as a result, recommendations have been made to reduce the risk in these patients. These recommendations include therapeutic lifestyle changes and, if treatment goals are not met, pharmacological intervention ( 1 ).Currently, few drugs are available that are suitable for treating patients with two components of the metabolic syndrome, insulin resistance and low HDL-C. Among treatments available for improving insulin resistance are the thiazolidinediones (TZDs) rosiglitazone and pioglitazone, which activate the nuclear receptor peroxisome proliferatoractivated receptor ␥ (PPAR ␥ ). In addition to their insulinsensitizing effects, TZDs have been shown to increase HDL-C levels in diabetic populations ( 2 ) by up to 14% for rosiglitazone and up to 19% for pioglitazone ( 3 ). These changes in HDL-C compare favorably to HDL-C changes achieved with other drugs currently approved to treat low HDL-C levels ( 4-7 ). Although rosiglitazone and pioglitazone both activate PPAR ␥ , they each have a characteristic metabolic response in regard to plasma lipid levels, with rosiglitazone also increasing plasma triglyceride levels in the relatively short term, whereas pioglitazone does not ( 8 ).The mechanisms responsible for the HDL-C-raising effects of TZDs are currently unknown. Studies that have measured apolipoprotein A-I (apoA-I) and apoA-II levels Abstract Treatment with the peroxisome proliferator-activated receptor ␥ agonist rosiglitazone has been reported to increase HDL-cholesterol (HDL-C) levels, although the mechanism responsible for this is unknown. We sought to determine the effect of rosiglitazone on HDL apolipoprotein A-I (apoA-I) and apoA-II metabolism in subjects with metabolic syndrome and low HDL-C. Subjects were treated with placebo followed by rosiglitazone (8 mg) once daily. At the end of each 8 week treatment, subjects (n = 15) underwent a kinetic study to measure apoA-I and apoA-II production rate (PR) and fractional catabolic rate. Rosiglitazone signifi cantly reduced fasting insulin and high-sensitivity C-reactive protein (hsCRP) and increased apoA-II levels. Mean apoA-I and HDL-C levels were unchanged following rosiglitazone treatment, although there was considerable individual variability in the HDL-C response. Rosiglitazone had no effect on apoA-I metabolism, whereas the apoA-II PR was increased by 23%. The change in HDL-C in response to rosiglitazone was signifi cantly correlated with the change in apoA-II concentration but not to changes in apoA-I, measures of glucose homeostasis, or hsCRP. Treatment with rosiglitazone signifi cantly increased apoA-II production in subjects with metabolic syndrome and low HDL-C but had no effect on apoA-I metabolism. The change in HDL-C in response to rosiglitazone treatment was u...