The level of urinary excretion of hydroxyproline is considered as an index of the metabolic activity of the collagen. It increases in situations which include an increase in the osteoblastic activity or in the bone resorption. In respiratory insufficiency a series of conditions occur which are theoretically capable of modyfing this parameter. Twelve patients (9 males and 3 women) with chronic respiratory disease in a situation of respiratory insufficiency (hypoxemia and/or hypercapnia at rest) were studied. The urinary excretion of hydroxyproline in these patients was 15.30 +/- 8.16 mg/day/m2, significantly greater than that of a control group with similar characteristics which was 9.97 +/- 3.07 mg/day/m2 (p less than 0.05; Student's t test). The existence of a significant correlation between the urinary excretion of hydroxyproline and the degree of hypoxemia (r = 0.66; p less than 0.01) was likewise verified; in the same way, although to a lesser degree with the hypercapnia (r = 0.62; p less than 0.05). The different factors capable of influencing the bone metabolism in respiratory insufficiency are discussed, as well as the effects of the medications used by these patients. It is possible, on the other hand, that the increase of the urinary excretion of hydroxyproline does not depend only on alterations in the metabolic condition of the bone, but also on a reduction in the hepatic metabolism of the amino acid in relation with gasometric modifications.