Although there are no standard guidelines for the treatment of autoimmune blistering diseases, azathioprine has shown good efficacy in acquired autoimmune blistering diseases, and is well tolerated. Side effects of azathioprine normally occur in mild variants. Severe reactions are due to reduced thiopurine S-methyltransferase (TPMT) or inosine triphosphate pyrophosphohydrolase (ITPA) activity. Therefore, screening for TPMT activity should be conducted in white patients and Africans, whereas Japanese should be screened for ITPA activity before therapy with azathioprine is started. Azathioprine is clinically meaningful for the treatment of pemphigus.