Among 173 consecutive open lung biopsies, nine gave a histopathological diagnosis of bronchiolitis. Seven of these patients had some connective tissue disorder (CTD), six of whom are presented in this report; two had classical and one possible rheumatoid arthritis (RA), one ankylosing spondylitis, one scleroderma, and one developed classical RA four years after biopsy. Four of the patients were smokers, most suffered from breathlessness and cough. In terms of lung function three patients had obstruction, one both restriction and obstruction and three a decreased diffusion capacity. For control purposes peripheral lung tissue was studied histologically from 24 consecutive smoking patients without CTD who underwent a lobectomy for cancer. Intraluminal plugs and mucosal lymphoplasmocytic infiltration of the bronchiolar walls were more prevalent and abundant in the CTD patients than in the controls (p less than 0.02 and p less than 0.001 respectively). Two CTD patients also showed some obliterative bronchiolitis. Corticosteroids were effective in one out of four patients treated. One patient improved and the others did not show any progression during the follow up. The results suggest that smoking alone does not explain the lesions of the small airways found in CTD patients, and that bronchiolitis may be specifically associated with the basic disorder in such cases.