The aim of the present study was to examine how liver markers are associated with insulin resistance in Japanese community-dwelling adults. This cross-sectional study included 587 men aged 58 ± 14 (mean ± standard deviation; range, 20-89) years and 755 women aged 60 ± 12 (range, 21-88) years. The study sample consisted of 998 (74.4%) non-obese body mass index (BMI) <25.0 kg/m2 and 344 (25.6%) overweight (BMI ≥ 25 kg/m2) subjects. Insulin resistance was defined by homeostasis model assessment of insulin resistance (HOMA-IR) of at least 2.5, and HOMA-IR and potential confounders were compared between the groups. Areas under the curve (AUC) of the receiver operating characteristic curves (ROC) were used to compare the power of these serum markers. In non-obese subjects, the best marker of insulin resistance was alanine aminotransferase (ALT)/aspartate aminotransferase (AST) ratio of 0.70 (95% confidence interval (CI), 0.63-0.77). In overweight subjects, AUC values for the ALT/AST ratio and ALT were 0.66 (0.59-0.72) and 0.66 (0.59-0.72), respectively. Multiple linear regression analyses for HOMA-IR showed that ALT/AST ratios were independently and significantly associated with HOMA-IR as well as other confounding factors in both non-obese and overweight subjects. The optimal cut-off point to identifying insulin resistance for these markers yielded the following values: ALT/AST ratio of ≥ 0.82 in non-obese subjects and ≥ 1.02 in overweight subjects. In non-obese subjects, the positive likelihood ratio was greatest for ALT/AST ratio. In non-obese Japanese adults, ALT/AST ratio may be the best reliable marker of insulin resistance.