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Although gamma-glutamyltransferase (GGT) is well known to be associated with metabolic syndrome, prospective data on longitudinal changes in GGT levels and incident cases of metabolic syndrome are limited. Therefore, we examined the association between changes in GGT levels over time and incident metabolic syndrome.A total of 2,579 Korean adults (913 men and 1,666 women; age range 40–70 years) free of metabolic syndrome were followed up for 2.6 years. Data were collected in 2005–2008 (baseline) and in 2008–2011 (follow-up). Serum GGT concentrations were determined by enzymatic methods.During an average of 2.6 years of follow-up, 558 participants (21.6%) developed metabolic syndrome. A gradual increase in the incidence of metabolic syndrome was observed across the GGT quartiles. In multivariate adjusted models, the odds ratio (95% confidence interval) for incident metabolic syndrome comparing the highest to lowest quartile of baseline GGT was 2.07 (1.52–2.80). The corresponding odds ratio for the highest GGT changes (>4 IU/L increase) with the lowest GGT changes (<-5 IU/L decrease) were 1.75 (1.32–2.33). The strength of this association was higher in participants with GGTs above the median compared with those with GGTs below the median.Both longitudinal increases in GGT and high baseline GGT, even levels within the reference range, are independent predictors of incident metabolic syndrome.