There is significant debate regarding high-density lipoprotein cholesterol (HDL-C) and high-fiber, low-fat diets. The present study was designed to examine the effects of lifestyle modification on the inflammatory/anti-inflammatory properties of HDL in obese men (n = 22) with metabolic syndrome factors. Subjects were placed on a high-fiber, low-fat diet in a 3-wk residential program where food was provided ad libitum and daily aerobic exercise was performed. Fasting blood was drawn pre- and postintervention for serum lipids, lipid hydroperoxides, and the ability of subject HDL to alter low-density lipoprotein (LDL)-induced monocyte chemotactic activity (MCA) in a human artery wall coculture. Induction of MCA by control LDL in the absence of HDL was normalized to 1.0. Values >1.0 after HDL addition indicated proinflammatory HDL; values <1.0 indicated anti-inflammatory HDL. In addition, proteins involved in regulating HDL function, apolipoprotein A-I (apoA-I), paraoxonase 1 and 3, and platelet-activating factor acetylhydrolase were measured. After 3 wk, decreases in total-cholesterol, LDL-cholesterol, HDL-C, triglycerides, total cholesterol-to-HDL cholesterol ratio, and lipid hydroperoxides (all P < 0.05) were noted. The HDL inflammatory index decreased (P < 0.05) from pro- (1.14 +/- 0.11) to anti-inflammatory (0.94 +/- 0.09). ApoA-I level and paraoxonase activity did not change; however, platelet-activating factor acetylhydrolase activity increased (P < 0.05). Despite a quantitative reduction in HDL-C, HDL converted from pro- to anti-inflammatory. These data indicate that intensive lifestyle modification improves the function of HDL even in the face of reduced levels, suggesting increased turnover of proinflammatory HDL.