Interstitial lung disease (ILD) develops in 30-50% of patients with polymyositis/dermatomyositis (PM/DM) and negatively affects their prognosis. The progression of PM/DM-ILD may be acute, subacute, chronic, or chronic becoming acute. The histopathological classification of PM/DM-ILD includes non-specific interstitial pneumonia (NSIP), organizing pneumonia (OP), diffuse alveolar damage (DAD), and usual interstitial pneumonia (UIP) or mixed variations. Some patients with acute/subacute interstitial pneumonia (A/SIP), typically with lung histology of OP or cellular NSIP, respond favorably to corticosteroid treatment, while others do not. Japanese patients with DM, especially those with clinically amyopathic DM (C-ADM) and palmar papules, seem to be at a greater risk of developing fulminant A/SIP with DAD histology resulting in pneumomediastinum and fatal outcome in a few months. An aggressive combination regimen including cyclosporine A (or tacrolimus) and cyclophosphamide should be immediately added to corticosteroid treatment for such patients. Sequential follow-up examination using high-resolution computed tomography (HRCT) of the chest and careful monitoring for bacterial and viral infections are essential. However, intensive immunosuppression alone may not be sufficient to control fulminant A/SIP, and other therapeutic targets, such as fibroblasts, should be considered.