The effect of lifestyle changes in cohorts of free-living populations has been surprisingly little evaluated. A longitudinal study. In the French Data from an Epidemiological Study on the Insulin Resistance (D.E.S.I.R) study of 1958 men and 2028 women, aged 30-65 years, the impact of 3-year changes in lifestyle habits (sporting activity, physical activity at home and at work, alcohol drinking, smoking) on metabolic syndrome parameters [insulin, glucose, high-density lipoprotein (HDL) cholesterol, triglycerides, systolic blood pressure, waist circumference] and on body mass index (BMI) were investigated. In men, 3-year increases in sporting activity were associated with a lowering of insulin, glucose, systolic blood pressure and waist circumference (all P < 0.05). For women, the only effect was on lowering waist circumference (P < 0.03). Increases in physical activity at home were beneficially associated with HDL-cholesterol, triglycerides, waist circumference and BMI changes (all P < 0.05) in men, but had no apparent effect in women. Decreases in alcohol intake only had an effect in men, with decreases in HDL-cholesterol and systolic blood pressure (P < 0.05), whereas decreasing cigarette smoking in men was associated with significant increases in insulin, glucose, triglycerides, waist and BMI (P < 0.001), and in women HDL-cholesterol, waist circumference and BMI increased (P < 0.02). These results were mainly caused by those who had stopped smoking. Increases in physical activity over the 3-year period were associated with beneficial effects on syndrome parameters, particularly in men. Smoking cessation and alcohol moderation produced mixed effects on these parameters.