PURPOSE Mac-2 binding protein (Mac-2 bp) is one of the major fucosylated glycoproteins, which we identified with glycol-proteomic analyses. We previously reported that fucosylated glycoproteins are secreted into bile, but scarcely secreted into sera in normal liver and hypothesized that the fucosylation-based sorting machinery would be disrupted in ballooning hepatocytes due to the loss of cellular polarity. In the present study, we investigated the availability of Mac-2 bp for differential diagnosis of nonalcoholic steatohepatitis (NASH) from nonalcoholic fatty liver disease (NAFLD) as a biomarker. EXPERIMENTAL DESIGN Serum Mac-2 bp levels were determined with our developed ELISA kit. Our cohort of 127 patients with NAFLD had liver biopsy to make a histological diagnosis of NASH and simple fatty liver. RESULTS Mac-2 bp levels were significantly elevated in NASH patients compared with non-NASH (simple steatosis) patients (2.132 ± 1.237 vs. 1.103 ± 0.500 μg/mL, p < 0.01). The area under the receiver-operating characteristic curve for predicting NASH by Mac-2 bp was 0.816. Moreover, multivariate logistic regression analyses showed Mac-2 bp levels could predict the fibrosis stage and the presence of ballooning hepatocytes in NAFLD patients. CONCLUSIONS AND CLINICAL RELEVANCE These results support the potential usefulness of measuring Mac-2 bp levels in clinical practice as a biomarker for NASH.