OBJECTIVES: Cardiovascular diseases are common causes of diseases and death for workers. With the increasing prevalence of obesity, the social costs for obesity related diseases are a growing burden in Korea. We aimed to investigate the impact of weight change on metabolic syndrome and its components in Korean male workers. METHODS: We analyzed the data from 2,785 male workers obtained during health checkups in 2000 and in 2008. The subjects were classified into 5 groups as Loss (< or = -3.0 kg), Stable (-2.9 to 2.9 kg), Mild gain (3.0 to 5.9 kg), Moderate gain (6.0 to 8.9 kg), and Severe gain (> or = 9.0 kg) group according to the amount of weight change between the 2 health checkups. The mean values of metabolic syndrome components were compared across the 5 weight change groups by ANCOVA. After classifying subjects into 2 groups according to the normality of their body weight at baseline, the odds ratios for metabolic syndrome and its components each weight-change group were computed by multiple logistic regression analyses, using the Stable group as the reference. RESULTS: There was a strong linear relationship between weight gain and a worsening of the components of metabolic syndrome in 2008 (p<0.001). In normal body weight group, the odds ratios for metabolic syndrome significantly increased in the Mild, Moderate, and Severe gain groups (OR 1.83, 2.82, and 7.56, respectively), and increased with weight gain (p<0.001). In subjects who were obese, the odds ratios for metabolic syndrome significantly increased if their amount of weight gain placed them in the Mild or Moderate gain groups(OR 1.75 and 3.97), increased with the increase of weight gain (p<0.001), and decreased in the Loss group(OR 0.51, 95%CI 0.30 to 0.85). CONCLUSIONS: Weight gain in male workers was positively associated with metabolic syndrome through worsening of metabolic syndrome components. On the other hand, weight loss in obese male workers had a protective effect against metabolic syndrome through the improvement of the components of metabolic syndrome.