Objective: Investigating the relationship between birth weight and the metabolic syndrome and the modifying effects of lifestyle in adults (36.5 years). Study Design and Setting: 273 subjects completed a birth-weight questionnaire; waist circumference, HDL and triglyceride concentrations, blood pressure and HbA1c, physical activity and fitness, smoking status and dietary intake were measured. Risks for the metabolic syndrome and for having at least two of the components of the metabolic syndrome were calculated and the modifying effect of lifestyle factors was assessed by comparing four groups based on birth weight and a lifestyle factor. Results: Birth weight was not associated with the metabolic syndrome (odds ratio OR = 0.67, 95% confidence interval CI = 0.25-1.79). Birth weight lower than the median increased the risk for having at least two components (OR = 2.28, 95% CI = 1.15-4.50). Subjects who smoked and had a birth weight lower than the median had a 6.9 times increased risk. Modification by lifestyle measured at age 36 was not observed, although effect modification using lifestyle data from 32 years was found. Conclusions: Lower birth weight increased the risk for having at least two components of the metabolic syndrome. Smoking, being less physical active or fit, have an unfavorable diet at age 32 adds to this risk. © 2005 Elsevier Inc. All rights reserved.