Most patients with nonalcoholic fatty liver disease (NAFLD) are overweight or obese. However, a significant proportion of patients have a normal body mass index (BMI), denoted as lean NAFLD. The long-term prognosis of lean NAFLD is unclear. We conducted a cohort study of 646 patients with biopsy-proven NAFLD. Patients were defined as lean (BMI < 25.0), overweight (BMI 25.0-29.9), or obese (BMI ≥ 30.0) at the time of biopsy. Each case was matched for age, sex, and municipality to 10 controls. Overall mortality and development of severe liver disease were evaluated using population-based registers. Cox regression models adjusted for age, sex, type 2 diabetes, and fibrosis stage were used to examine the long-term risk of mortality and liver-related events in lean and nonlean NAFLD. Lean NAFLD was seen in 19% of patients, while 52% were overweight and 29% were obese. Patients with lean NAFLD were older, had lower transaminases, lower stages of fibrosis, and lower prevalence of nonalcoholic steatohepatitis at baseline compared to patients with a higher BMI. During a mean follow-up of 19.9 years (range 0.4-40 years) representing 12,631 person years and compared to patients who were overweight, patients with lean NAFLD had no increased risk for overall mortality (hazard ratio 1.06; 0.73) while an increased risk for development of severe liver disease was found (hazard ratio 2.69; 0.007). : Although patients with lean NAFLD have lower stages of fibrosis, they are at higher risk for development of severe liver disease compared to patients with NAFLD and a higher BMI, independent of available confounders. ( 2018;2:48-57).