To evaluate the significance of the association of malignancy with autoimmune blistering diseases, we studied the incidence of internal malignancies in pemphigus and bullous pemphigoid based upon 496 cases of pemphigus and 1113 cases of bullous pemphigoid in Japan. Results showed that (1) an association between internal malignancies and pemphigus was observed in 25 out of 496 cases (5.0%), while that with bullous pemphigoid was seen in 64 out of 1113 cases (5.8%). Such association ratios were significantly higher than that of the controls aged over 70 years old (0.61%); (2) The average ages of pemphigus/bullous pemphigoid with malignancy were 64.7 and 69.2 years, respectively. The association ratio of malignancy with pemphigus increased by age, while that with pemphigoid was not correlated with aging; (3) Lung cancer was most common in pemphigus and gastric cancer in bullous pemphigoid; (4) There were no significant differences in the titers of circulating antibody, the presence or extent of mucous involvement or annular erythema between bullous pemphigoid patients with malignancy and without malignancy. Our results indicated that detailed examination for internal malignancy is essential for those patients with pemphigus or bullous pemphigoid.