Fatty liver disease (FLD) is a condition where large vacuoles of triglyceride fat accumulate in liver cells through the process of steatosis.

Despite having multiple causes, FLD can be considered a single disease that occurs worldwide in those with excessive alcohol intake and those who are obese (with or without effects of insulin resistance). The condition is also associated with other diseases that influence fat metabolism.

Most individuals do not exhibit symptoms, and are usually discovered by chance because of abnormal liver function tests or hepatomegaly noticed in an unrelated medical condition. Patients may complain of fatigue, malaise and dull right upper quadrant abdominal discomfort; yet these symptoms are experienced less frequently.

Non-alcoholic fatty liver disease (NAFLD) is fatty inflammation of the liver when this is not due to excessive alcohol use. It is related to insulin resistance and the metabolic syndrome, and may respond to treatments originally developed for other insulin resistant states (e.g. diabetes mellitus type 2), such as weight loss.