Interstitial lung disease (ILD) has been recognized as a manifestation of polymyositis or dermatomyositis (PM/DM). Patients with PM/DM with anti-Jo-1 antibody had a higher incidence of ILD. Thirty-five patients with PM/DM were studied the antibodies. Six (16.1%) were found to have them. ILD occurred in all 6 anti-Jo-1 positive patients, as opposed to 10 (34.5%) of 29 anti-Jo-1 negative patients. Only 2 cases of undifferentiated connective tissue disease among 1320 patients with various kinds of collagen vascular diseases other than PM/DM were positive for anti-Jo-1 antibody. These two patients also had lung involvement. Among six PM/DM patients with anti-Jo-1 antibody, dyspnea preceded proximal muscle weakness in three patients. In one case, the anti-Jo-1 antibody had been detected one month prior to the onset of myositis. Anti-Jo-1 antibody is a useful marker for PM/DM with ILD.