Previous studies have suggested that the hypoxia and/or hypercapnia associated with chronic lung diseases may lead to pituitary and gonadal dysfunction, with destruction of the sella turcica. It is unclear, however, whether these abnormalities were due to lung disease or to confounding factors. We studied the relationships between hormonal levels (triiodothyronine, thyroxine, T3 resin uptake, thyrotropin, prolactin, cortisol, and testosterone) and PaO2, PaCO2, pH, and alveolar-arterial gradient in 25 patients with chronic lung disease. These patients were highly homogeneous for diagnosis, age, sex, ambulatory status, lack of other illnesses, and minimal use of medications unrelated to lung disease, but did have various degrees of hypoxia and hypercarbia at the time of study. We found no relationship between hormonal levels and lung function, or evidence of major pituitary involvement on lateral roentgenograms of the skull, CT of the sella turcica, or stimulation of the pituitary. An inverse correlation did occur between serum levels of thyroxine and the daily dose of oral prednisone. We conclude that most of the endocrine dysfunction ascribed to chronic lung diseases is probably due to factors other than hypoxia or hypercarbia.