Aim To investigate the risk factors for interstitial lung disease (ILD) and prognosis in patients with idiopathic inflammatory myopathy (IIM). Methods A retrospective longitudinal study was performed in patients diagnosed with IIM between January 2012 and December 2018. Results The study cohort included 91 men and 195 women who were classified as having dermatomyositis (DM, n = 183), polymyositis (PM, n = 77), or clinical amyopathic DM (CADM, n = 26). ILD was identified in 46.5% (n = 133) of patients with IIM. The independent risk factors for ILD were age at disease onset, presence of anti-Ro-52 antibody, Gottron's papules, elevated serum immunoglobulin M levels and hypoalbuminemia. Older age at disease onset, ILD, malignancy, and increased serum aspartate aminotransferase and neutrophil-to-lymphocyte ratio (NLR) were identified as the independent predictors for mortality, whereas elevated serum albumin level was associated with a better prognosis. A total of 73 deaths (25.5%) occurred after a median follow-up time of 33 months. Infection (49.3%) was the leading cause of death. In the overall cohort, the 1-year, 5-year and cumulative survival rates were 83.2%, 74.2% and 69.4%, respectively. The receiver operating characteristic curve indicated that the optimal cut-off value of NLR for predicting death in IIM was 6.11. Conclusion IIM patients have a poor prognosis with substantial mortality, especially in patients who have older age at onset, ILD, malignancy and higher NLR. Close monitoring and aggressive therapies are required in patients having poor predictive factors.