['Serum alanine aminotransferase (ALT) activity is a widely-used surrogate marker for liver injury. However, mild elevation of serum ALT is frequently observed in apparently healthy individuals, making it sometimes challenging to interpret whether this laboratory abnormality is medically benign or serious. To obtain a better understanding of the factors influencing ALT levels, we examined the relation between ALT and a number of anthropometric and biochemistry measurements in humans.', 'We assessed the associations of ALT with hematocrit (HCT) in 1,200 apparently healthy adults from an Amish population. Multivariate analyses were carried out to determine whether observed associations were independent of other factors known to modulate ALT and HCT, including body mass index (BMI) and sex. The correlation detected in the Amish was then replicated in an independent population sample (N = 9,842) from the National Health and Nutrition Examination Survey (NHANES) III.', 'ALT levels were positively correlated with HCT (r = 0.33, p < 0.0001) in both Amish and NHANES III. The magnitude of association was unchanged after adjustment for BMI, but was reduced by age/sex adjustment to r = 0.18 (p < 0.0001) and r = 0.17 (p < 0.0001) in the Amish and NHANES populations, respectively. HCT accounts for about 3% of the population variation in ALT, which is smaller than the contributions of gender and BMI, but larger than individual blood pressure and cholesterol components.'] Is serum alanine aminotransferase correlated with hematocrit in healthy human subjects?