We encountered a pair of monozygotic twins with summer-type hypersensitivity pneumonitis. Patients 1 and 2 were 24-year-old men who worked in the same place and shared the same room, which was built of wood. In August, patient 1, a non-smoker, was referred to our hospital because of coughing, fever and dyspnea. In November, the same symptoms appeared in patient 2, who was a smoker. The difference between the onset times in patients 1 and 2 was suspected to be due to cigarette smoking, because their occupations and other circumstances were the same and they were monozygotic twins.