Chronic upper respiratory tract diseases are severe problems in captive orangutans (Pongo abelii, P. pygmaeus), but the etiology and pathogenesis are unknown. This study recorded the prevalence of such diseases in captive European orangutans (201 animals; 20 zoos) and investigated possible predisposing factors. Bornean orangutans (P. pygmaeus) showed chronic respiratory signs (13.8% of all animals) significantly more often than Sumatran (P. abelii; 3.6%), and male animals more often (15.8%) than females (3.9%). Hand-reared animals developed more air sacculitis (21%) than parent-reared animals (5%). Diseased animals were more often genetically related to animals with respiratory diseases (93%) than to healthy animals (54%). None of the environmental conditions investigated (enclosure size and structure, climate, human contact) had a significant effect on disease prevalence. The present results suggest a higher importance of individual factors, including inheritance, for the development of upper respiratory tract diseases than environmental conditions. Bornean, male and hand-reared orangutans and animals related to diseased animals need increased medical surveillance for an early detection of respiratory diseases at a possibly still curable stage.