Myotonic dystrophy (DM) is an autosomal dominant multisystem disorder. Little evidence suggests the existence of liver damage in a small number of patients. We have prospectively evaluated liver and gallbladder function in 53 patients with DM in relation to clinical and genetic parameters. None of the patients had an enlarged liver, signs of cirrhosis, or portal hypertension. All were free of medication, and none were pregnant or had a history of alcohol abuse. In 35 (66%) patients, serum activity of at least one of six liver enzymes assayed was abnormal. An elevated level of alkaline phosphatase was found in 50.9%, of gamma-glutamyltransferase in 52.8%, of 5′ nucleotidase in 43.4%, of serum aspartate aminotransferase in 35.8%, of serum alanine aminotransferase in 33.9%, and of lactate dehydrogenase in 37.7%. Liver function test results did not correlate with severity of muscle weakness, disease duration, or serum levels of creatine kinase, glucose, or lipids. Motility of gallbladder and abdominal ultrasonography were normal. Cytosine-thymidine-guanine repeat expansion by southern blot did not correlate with liver enzyme abnormalities. We conclude that elevation of liver enzymes is frequent in DM and should be included as an additional laboratory finding of the disease.