We studied 32 patients with interstitial pneumonia associated with collagen disease. All underwent surgical lung biopsy. Twenty-seven were histologically classified as follows: 8 with usual interstitial pneumonia (UIP), 10 with nonspecific interstitial pneumonia (NSIP), 6 with bronchiolitis obliterans organizing pneumonia (BOOP), and 3 with diffuse alveolar damage (DAD). Twenty-five of the patients were treated with corticosteroid but 4 (3 DAD, 1 NSIP) deteriorated rapidly and died within a month after lung biopsy. At the final follow-up, all 8 UIP patients were alive with residual respiratory impairment, whereas 6 NSIP and 4 BOOP patients had almost completely recovered. These findings suggested that histologic classification can be highly valuable to the therapeutic responsiveness and prognosis in cases of interstitial pneumonia associated with collagen disease.