The metabolic syndrome is known to sometimes exist in the presence of normal aminotransferase levels. The purpose of this study was to determine the lowest sex-specific level of alanine aminotransferase associated with the metabolic syndrome in a nationwide, representative Korean population. We analyzed data from adults 20 years and older (n = 3405) assessed in the Third Korean National Health and Nutrition Examination Survey (2005). Participants were divided into 4 groups according to the quartiles of alanine aminotransferase levels for each sex. Logistic regression modeling was performed after adjustment for age, body mass index, waist circumference, smoking, ingested alcohol amount, and physical activity. Alanine aminotransferase level groups 3 and 4 in women (> or =15 IU/L) and group 4 in men (> or =27 IU/L) were significantly associated with the metabolic syndrome compared with the lowest alanine aminotransferase groups (<16 IU/L in men, <11 IU/L in women). In men, the odds ratio (95% confidence interval) of the metabolic syndrome was 2.71 (1.31-5.63) for alanine aminotransferase group 4 (> or =27 IU/L). In women, odds ratios were 1.69 (1.02-2.80) and 2.06 (1.23-3.43) for alanine aminotransferase groups 3 (15 < or = alanine aminotransferase < 19 IU/L) and 4 (> or =19 IU/L), respectively. High-normal alanine aminotransferase levels (> or =27 IU/L in men, > or =15 IU/L in women) were strongly associated with the metabolic syndrome in Korean adults.