BACKGROUND AND AIMS: Accurate diagnosis of non-alcoholic steatohepatitis (NASH) from non-alcoholic fatty liver disease (NAFLD) is clinically important. Therefore, there is a need for easier ways of diagnosing NASH. In this study, we investigated the serum fatty acid composition and evaluated the possibility of using the serum fatty acid composition as a diagnostic marker of NASH. METHODS: The subjects were 78 NAFLD patients (non-alcoholic fatty liver [NAFL]: 30, NASH: 48) and 24 healthy individuals. Fatty acids extracted from the liver tissue and serum were identified and quantified by gas chromatography. In addition, we evaluated the relationship between serum and liver tissue fatty acid composition, patient background, and liver histology. The diagnostic performance of NASH was evaluated by calculating the area under the receiver operating characteristic (AUROC). RESULTS: The results of the fatty acid analysis showed the C16:1n7/C16:0 ratio to have the strongest correlation between serum and liver tissue (r = 0.865, P < 0.0001). The serum C16:1n7/C16:0 ratio in the NASH group was higher compared with that in the NAFL group (P = 0.0007). Evaluation of the association of the serum C16:1n7/C16:0 ratio with liver histology revealed significant correlation with lobular inflammation score, ballooning score, and fibrosis score. The AUROC for predicting NASH in all NAFLD patients was 0.7097. The AUROC was nearly equivalent even when the study subjects were restricted to patients with a fibrosis score ≤ 2 only (AUROC 0.6917). CONCLUSION: Measuring the serum C16:1n7/C16:0 ratio may be an effective non-invasive method for diagnosing NASH, particularly in its early stages.