Cancer incidence and mortality risks were evaluated in a combined cohort of patients who were hospitalized for porphyria in Denmark (1977-1989) and Sweden (1965-1983). Patients were identified by using population-based hospitalization registries. The unique individual identification numbers of 530 patients with porphyria cutanea tarda (PCT) and 296 with acute intermittent porphyria (AIP) were linked to the nationwide cancer and death registries. Among patients with both types of porphyria, the authors found small but significantly elevated risks of all cancers combined (PCT: standardized incidence ratio (SIR) = 1.7, 95% confidence interval (CI) 1.3-2.2; AIP: SIR = 1.8, 95% CI 1.1-2.8) due to pronounced excesses of primary liver cancer (PCT: SIR = 21.2, 95% CI 8.5-43.7; AIP: SIR = 70.4, 95% CI 22.7-164.3) and moderate increases in lung cancer (PCT: SIR = 2.9, 95% CI 1.5-5.2; AIP: SIR = 2.8, 95% CI 0.3-10.2). PCT patients had a significantly increased risk of mortality from liver cirrhosis (standardized mortality ratio (SMR) = 8.4, 95% CI 3.1-18.4) or chronic obstructive pulmonary disease (SMR = 3.1, 95% CI 1.1-6.7). The increased risk of primary liver cancer and the increased risk of mortality from cirrhosis of the liver are consistent with findings from previous clinical surveys, but the new observations of excess lung cancer and chronic obstructive pulmonary disease require confirmation.