Nonalcoholic fatty liver disease (NAFLD) is the most common cause of elevated liver enzymes in patients of developed countries. We determined the long‐term clinical and histological courses of such patients. In a cohort study, 129 consecutively enrolled patients diagnosed with biopsy‐proven NAFLD were reevaluated. Survival and causes of death were compared with a matched reference population. Living NAFLD patients were offered repeat liver biopsy and clinical and biochemical investigation. Mean follow‐up (SD) was 13.7 (1.3) years. Mortality was not increased in patients with steatosis. Survival of patients with nonalcoholic steatohepatitis (NASH) was reduced (P = .01). These subjects more often died from cardiovascular (P = .04) and liver‐related (P = .04) causes. Seven patients (5.4%) developed end‐stage liver disease, including 3 patients with hepatocellular carcinoma. The absence of periportal fibrosis at baseline had a negative predictive value of 100% in predicting liver‐related complications. At follow‐up, 69 of 88 patients had diabetes or impaired glucose tolerance. Progression of liver fibrosis occurred in 41%. These subjects more often had a weight gain exceeding 5 kg (P = .02), they were more insulin resistant (P = .04), and they exhibited more pronounced hepatic fatty infiltration (P = .03) at follow‐up. In conclusion, NAFLD with elevated liver enzymes is associated with a clinically significant risk of developing end‐stage liver disease. Survival is lower in patients with NASH. Most NAFLD patients will develop diabetes or impaired glucose tolerance in the long term. Progression of liver fibrosis is associated with more pronounced insulin resistance and significant weight gain. (HEPATOLOGY 2006;44:865–873.)