BACKGROUND: The relationship of lifestyle and socioeconomic factors with weight change over 4 years in Korean middle-aged group was assessed. METHODS: A total of 1,167 men, aged 40~59 years, were selected from the Health Promotion Center of Ulsan University Hospital during the period of Jan. 2000 through Mar. 2000. Including anthropometric measures, designed questionnaires were administered concern lifestyle behavior topics with cigarette smoking, exercise, alcohol consumption, dairy product consumption and socioeconomic factors of monthly income and education. The subjects were divided into three groups by body mass index (BMI: kg/m(2)); normal (<23 kg/m(2)), overweight, and obesity (> or =25 kg/m(2)). After 4 years of follow-up, the men were classified into weight change categories; 1) stable weight group (from normal to normal, from overweight to overweight) and 2) weight gain group (from normal to overweight/obesity, from overweight to obesity). RESULTS: The weight gain group were 18.4% (n=215) and the stable overweight group were 81.6% (n=952). Adjusted for age, the frequency of dairy product consumption, the frequency of alcohol drinking, the regularity of exercise, education and monthly income, the smokers (> or =10/day) had significantly high incidence rates of increase in weight (odds ratio=1.77, 95% CI 1.07~2.90). The increased dairy consumers (> or =4/week) had significantly low incidence rates of increase in weight (odds ratio=0.43, 95% CI 0.20~0.93). Otherwise, there were no significant relationships in weight change. CONCLUSION: Smoking and decreased dairy product consumption were related to the increase in weight.