To evaluate the clinical feature of pulmonary fibrosis and of the patients with the an tibodies to 2 types of intermediate filaments (IFs), vimentin and cytokeratin filaments in mixed connective tissue disease (MCTD), we studied 44 patients who were followed at our hospital over 2 years. Pulmonary fibrosis was diagnosed in 43% of MCTD patients by chest X ray and computed tomography. The intensities of pulmonary fibrosis in most of patients were not severe. Pa02 levels of patients with pulmonary

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... fibrosis were significantly lower than those without it, although there were no difference of values of vital capacity and diffusing capacity (DLco) between patients with and without pulmonary fibrosis. MCTD patients with pulmonary fibrosis had frequently proximal scleroderma, but fewer components of systemic lupus erythematosus (SLE). The antibodies to vimentin and cytokeratin filaments, were assayed by western blotting in sera from 39 patients. The antibody to vimentin from HEL-299 cell was detected in sera of 56% and the antibody to cytokeratin from PtK-2 cell was present in 15% of patients' sera. These antibodies belonged mainly to IgG class. No healthy volunteer had anti-IFs antibody. The frequencies of proximal scleroderma, pulmonary hypertension and pulmonary fibrosis in patients with the antibody to vimentin were higher than the patients without it. This result suggests the possibility that the antibody to vimentin filament is associated with the pul monary involvement in MCTD.