Background: Porphyria cutanea tarda (PCT) is a rare hepatocutaneous disease for which the prognosis is largely unknown. Objective: To compare all-cause and cause-specific mortality between a nationwide cohort of patients with PCT and a matched population sample. Methods: We included all Danish patients who received a diagnosis of PCT from 1989 through 2012. Each patient was matched by age and sex to 10 random population control individuals. We compared survival and cause-specific mortality between patients and control individuals and adjusted for confounding from alcohol-related diseases, hepatitis, hemochromatosis, HIV, diabetes, acute myocardial infarction, stroke, cancer, chronic obstructive pulmonary disease, and cirrhosis. Results: The 20-year survival was 42.9% (95% confidence interval [CI], 36.9-48.7) for patients with PCT compared with 60.5% (95% CI, 58.6-62.4) for matched control individuals. All-cause mortality hazard ratio (HR) was 1.80 (95% CI, 1.56-2.07) before adjustment and 1.22 (95% CI, 1.04-1.44) after adjustment. The cause-specific mortality was markedly increased for nonmalignant gastrointestinal diseases (HR, 5.32; 95% CI, 2.71-10.43) and cancers of the gut (HR, 2.05; 95% CI, 1.24-3.39), liver/gallbladder (HR, 11.24; 95% CI, 4.46-28.29), and lungs (HR, 2.17; 95% CI, 1.41-3.33). Limitations: We had no data on lifestyle factors. Conclusions: Patients with PCT have increased mortality, primarily explained by an increased mortality from gastrointestinal diseases and from cancers of the gut, liver/gallbladder, and lungs.