The glucose tolerance was evaluated in 136 nonalcoholic fatty liver disease (NAFLD) patients without overt diabetes mellitus. All patients underwent liver biopsies and 75-g oral glucose tolerance tests. Plasma glucose and insulin levels were analyzed periodically for 3 h after oral glucose loading. Irrespective of the hemoglobin A1c levels, impaired glucose tolerance, including diabetes mellitus, was detected in 61% of the NAFLD patients. While the secretion pattern of glucose after glucose loading was similar among the NAFLD patients, postprandial insulin levels increased in parallel with the aggravation of histological findings (fibrosis stages and NAFLD activity scores). In conclusion, the present data shows the importance of performing the glucose tolerance test in NAFLD patients without overt diabetes mellitus.