The aim of the study was to estimate the prevalence, characteristics, and prognostic factors of interstitial lung disease (ILD) in patients with polymyositis (PM) and dermatomyositis (DM). The medical records of 151 PM/DM patients treated at Chang Gung Memorial Hospital between January, 2000 and June, 2007 were retrospectively reviewed. Thirty of 151 (19.9%) PM/DM patients had developed ILD. Older age at PM/DM onset, anti-Jo-1 antibody, and arthritis/arthralgia were associated with the presence of ILD (p = 0.004, p = 0.008, and p = 0.026, respectively). Anti-Jo-1 was initially excluded from the multivariate analysis because only 80 patients underwent the test. An older age at onset above 45 years (odds ratio 3.28, 95% confidence interval (CI) 1.15-9.34, p = 0.026) and arthritis/arthralgia at onset (odds ratio (OR) 2.57, 95% CI 1.09-6.08, p = 0.032) were the two independent risk factors for developing ILD. If anti-Jo-1 was included in the multivariate analysis (n = 80), then an older age at onset above 45 years (OR 7.30, 95% CI 1.70-31.40, p = 0.008) and anti-Jo-1 positive (OR 7.89, 95% CI 1.18-52.87, p = 0.033) were associated with ILD, while arthritis/arthralgia was no longer significant (OR 2.64, 95% CI 0.70-10.01, p = 0.153). Of the 30 ILD patients, 16 (53.3%) died. The survival time was significantly shorter in ILD patients than in patients without ILD (p < 0.001). Poor survival in ILD patients was associated with male gender (p = 0.039), a Hamman-Rich-like presentation (p = 0.039), and a clinical diagnosis of acute interstitial pneumonia (p = 0.007).