A recent study in young Italian subjects suggested that the healthy thresholds for serum alanine aminotransferase (ALT) levels should be adjusted to 30 IU/L for men and 19 IU/L for women when assessing risk factors for nonalcoholic fatty liver disease. Our aim was to assess serum ALT concentrations in healthy Korean individuals and to determine the factors affecting ALT levels in these populations. We included 1,105 potential liver donors (643 men and 462 women) with biopsy-proven normal livers. Median ages were 25 years in men and 30 years in women, with a median body mass index (BMI) of 22.3 kg/m2 in men and 21.4 kg/m2 in women. The calculated thresholds for ALT values in these subjects were 35 IU/L for men and 26 IU/L for women. Age and BMI were independently correlated with ALT levels in both sexes, whereas serum total cholesterol concentration was significant only in men and blood glucose level only in women (P < 0.05). When we chose a subgroup of 665 individuals (346 men and 319 women) using Prati criteria, modified by the BMI cutoff points for Asians (<23 kg/m2), we found that the healthy ALT values were 33 IU/L for men and 25 IU/L for women. The mean ALT concentrations for subjects within the Prati criteria were significantly lower than for those outside the criteria (16.7 versus 19.5 IU/L for men, 12.8 versus 14.9 IU/L for women; P < 0.001). Conclusion: The healthy ALT thresholds in biopsy-proven normal Asians were clearly lower than the previously accepted thresholds, as has also been noted in Europeans. Age, BMI, and/or other metabolic parameters significantly affect ALT levels, even in subjects with normal livers. (HEPATOLOGY 2010.)