AIM: To evaluate the relation between type of dietary fatty acid and degree of insulin resistance. DESIGN: A cross-sectional study. METHODS: Anthropometrical data were measured in 538 subjects, aged 18-65 Years, selected randomly from the municipal census of Pizarra (Spain). An oral glucose tolerance test (OGTT) was given to all subjects and measurements were made of glycemia, insulinemia and the proportion of fatty acids in plasma phospholipids. Insulin resistance (IR) was estimated by homeostasis model assessment. Samples of cooking oil being used were obtained from the kitchens. The strength of association between variables was measured by calculating the odds ratio (OR) from logistic models, and the relationships were measured by linear correlation coefficients. RESULTS: Insulin resistance was significantly less in people who used olive oil compared with those who used sunflower oil or a mixture. Statistical significance remained in the group of people with normal OGTT after adjusting for obesity. In the whole sample, IR correlated negatively with the concentration of oleic acid (r=-0.11; P=0.02) and positively with that of linoleic acid (r=0.10; P=0.02) from the cooking oil. In subjects with normal OGTT, IR correlated negatively with oleic acid from cooking oil (r=-0.17; P=0.004) and from plasma phospholipids (r=-0.11; P=0.01) and positively with the concentration of linoleic acid in cooking oil (r=0.18; P=0.004) and plasma phospholipids (r=0.12; P=0.005). The risk (OR) of having raised IR was significantly lower in people who consumed olive oil, either alone (OR=0.50) or mixed (OR=0.52) compared with those who consumed only sunflower oil. CONCLUSION: There is an association between the intake of oleic acid, the composition of oleic acid in plasma phospholipids and peripheral insulin action.