The activity of serum gamma-glutamyltransferase (gamma-GT) and its determinants were studied in 17 patients with myotonic dystrophy. The gamma-GT activity was elevated in 11 patients and its mean value was five-fold higher than in healthy controls. The increase in gamma-GT could not be explained by factors generally known to result in a misleading elevation of gamma-GT. Most patients with elevated gamma-GT also had one or more other pathological laboratory tests related to hepatic function but none had a clinically significant liver disease. Serum gamma-GT activity was not related to the disability caused by dystrophy or to the level of serum creatine kinase suggesting that the elevation of serum gamma-GT is not an indication of a general cell membrane dysfunction. It is concluded that the increase in serum gamma-GT activity in patients with myotonic dystrophy is due to a real but mild liver involvement, which should be taken into account in the examination of these patients who often complain of gastrointestinal symptoms.