1 Central Arkansas Veterans Healthcare System, 2 Division of Endocrinology, Department of Medicine, and 3 Department of Pediatrics and Winthrop P. Rockefeller Cancer Institute, University of Arkansas for Medical Sciences, Little Rock, Arkansas; 4 Department of Biostatistics, University of Wisconsin, Milwaukee, Wisconsin; and 5 College of Health Sciences, University of Kentucky, Lexington, Kentucky Submitted 30 October 2008 ; accepted in final form 18 March 2009 Obesity is characterized by adipose tissue expansion as well as macrophage infiltration of adipose tissue. This results in an increase in circulating inflammatory cytokines and nonesterified fatty acids, factors that cause skeletal muscle insulin resistance. Whether obesity also results in skeletal muscle inflammation is not known. In this study, we quantified macrophages immunohistochemically in vastus lateralis biopsies from eight obese and eight lean subjects. Our study demonstrates that macrophages infiltrate skeletal muscle in obesity, and we developed an in vitro system to study this mechanistically. Myoblasts were isolated from vastus lateralis biopsies and differentiated in culture. Coculture of differentiated human myotubes with macrophages in the presence of palmitic acid, to mimic an obese environment, revealed that macrophages in the presence of palmitic acid synergistically augment cytokine and chemokine expression in myotubes, decrease I B- protein expression, increase phosphorylated JNK, decrease phosphorylated Akt, and increase markers of muscle atrophy. These results suggest that macrophages alter the inflammatory state of muscle cells in an obese milieu, inhibiting insulin signaling. Thus in obesity both adipose tissue and skeletal muscle inflammation may contribute to insulin resistance. muscle cells; inflammation Address for reprint requests and other correspondence: C. A. Peterson, College of Health Sciences. Univ. of Kentucky, Lexington, KY 40536 (e-mail: cpete4{at}uky.edu )