Pigeon breeders are exposed to high levels of avian antigens that are potent causes of hypersensitivity pneumonitis but little is known about the long term risk of chronic interstitial lung disease. The study combines detailed and well maintained records on all breeders of racing pigeons in Denmark since 1901 with national health registers. From this data we can study the long-term risk by “pigeon years” and other exposure metrics that may provide new insight to the causes of interstitial lung diseases.