Recent studies suggest that insulin sensitivity is related to the fatty acid composition of phospholipids in skeletal muscle (SM) membranes. Since it is difficult to obtain muscle biopsies, it may be useful to have information on the fatty acid composition using more accessible cells such as erythrocytes. This would be possible only if the composition of erythrocyte and muscle membranes are very similar. Since no comparative data are available, we evaluated the phospholipid fatty acid composition of erythrocyte and SM membranes in 16 individuals, 10 nondiabetics (male to female ratio, 4:6; age, 50 +/- 11 years; body mass index, 27 +/- 5 kg/m2; mean +/- SD) and 6 type 2 diabetic patients (male to female ratio, 2:4; age, 64 +/- 5 years; body mass index, 27 +/- 4 kg/m2). All patients underwent abdominal surgery, during which a biopsy of the abdominal rectus muscle (50 to 100 mg) was obtained. Erythrocyte and SM phospholipid fatty acids were extracted and then methylated; the methyl fatty acids were separated and quantified by gas chromatography. Compared with erythrocyte membranes, muscle membranes showed a significantly higher proportion of omega-6 polyunsaturated fatty acid ([PUFA] 43.0% +/- 3.1% v29.7% +/- 1.6%, P < .001) and lower saturated fatty acid ([SFA] 41.1% +/- 1.5% v 43.4% +/- 1.2%, P < .001), monounsaturated fatty acid ([MUFA] 11.5% +/- 1.7% v 20.0% +/- 1.9%, P < .001), and omega-3 PUFA (3.8% +/- 0.6% v 7.4% +/- 1.0%, P < .001). The greatest increase involved linoleic acid (26.9% +/- 2.8% v 10.3% +/- 1.6%, P < .001), whereas lignoceric acid (0.8% +/- 0.2% v 5.0% +/- 0.6%, P < .001) and oleic acid (10.4% +/- 1.6% v 13.5% +/- 1.3%, P < .001) were significantly lower. These results show that erythrocyte and muscle membrane phospholipid fatty acids are significantly different. Therefore, data on SM membranes cannot be extrapolated on the basis of measures of erythrocyte phospholipid fatty acid composition.