We studied lung collagen metabolism in 18 patients with hypersensitivity pneumonitis to determine if changes at this level could explain the different clinical courses followed by these patients. Collagen concentration, biosynthesis and degradation were measured in lung tissue samples obtained before treatment. Four patients healed, eight improved and six did not improve or worsened. All patients who healed showed an important increase in collagenolysis; patients who improved had normal or high values, but significantly less than those obtained in patients who healed. Finally, five out of the six patients who did not improve or worsened had a significant decrease in degradation. These findings support the notion that a diminution of local collagenolysis may play a role in the progression to fibrosis in some patients with hypersensitivity pneumonitis and can also be a useful tool to predict the prognosis of this disease.