BACKGROUND:The uncoupling protein UCP1 has been related to the development and/or maintenance of obesity through its involvement in regulating energy balance. The role of this mitochondrial protein in humans is uncertain due to the scarce presence of the brown adipose tissue in the adult individual. The polymorphism -3826 A/G of the UCP1 alone or in conjunction with the mutation Trp64Arg of the adrenergic receptor b3 has been associated with obesity, diabetes mellitus and related diseases although with contradictory results. With the aim of determining the influence of polymorphism -3826 A/G of the UCP1 on the classical components of the metabolic syndrome in our population, we studied 159 obese individuals and 154 of normal weight, with a study design of cases and controls. In all of them Body Mass Index (BMI), hip/waist index, % of body fat, arterial tension (AT), lipidic profile, leptine, basal glucemia and basal insulinemia were determined. Similarly, the presence of the above mentioned mutation of the UCPI gene was analysed. RESULTS:Significant differences were obtained in all of the variables studied between obese (cases) and normal weight (controls). Within the obese group, polymorphism -3826 A/G of the UCP1 gene (n=53) was associated with a greater BMI (p=0.03), greater percentage of body fat (p=0.04) and higher AT both systolic (p=0.009) and diastolic (p=0.02). There were no statistically significant differences in any of the other indices evaluated. CONCLUSION:The fundamental factor that influences the components of the metabolic syndrome is obesity. However, the polymorphism -3826 A/G of the UCP1 gene is associated with a greater degree of obesity and very high figures of AT.