The cellular basis of insulin resistance is still unknown, however, relationships have been demonstrated between insulin action in muscle and the fatty acid profile of the major membrane structural lipid (phospholipid). The present study aimed to further investigate the hypothesis that insulin action and adiposity are associated with changes in the structural lipid composition of the cell. In 52 adult male Pima Indians, insulin action (euglycemic clamp), percentage body fat (pFAT; underwater weighing), and muscle phospholipid fatty acid composition (percutaneous biopsy of vastus lateralis) were determined. Insulin action (high-dose clamp; MZ) correlated with composite measures of membrane unsaturation (% C20-22 polyunsaturated fatty acids [r = 0.463, P < 0.001], unsaturation index [r = -0.369, P < 0.01]), a number of individual fatty acids and with ??5 desaturase activity (r = 0.451, P < 0.001). pFAT (range 14-53%) correlated with a number of individual fatty acids and ??5 desaturase activity (r = -0.610, P < 0.0001). Indices of elongase activity (r = -0.467, P < 0.001), and ??9 desaturase activity (r = 0.332, P < 0.05) were also related to pFAT but not insulin action. The results demonstrate that ??5 desaturase activity is independently related to both insulin resistance and obesity. While determining the mechanisms underlying this relationship is important for future investigations, strategies aimed at restoring 'normal' enzyme activities, and membrane unsaturation, may have therapeutic importance in the 'syndromes of insulin resistance.'