Autoantibodies against melanoma differentiation-associated gene 5 (MDA5) are important serological markers in dermatomyositis (DM) with rapidly progressive interstitial lung disease (ILD). Recent studies noted that anti-MDA5 antibody (anti-MDA5ab), ferritin, and IL-18 are useful biomarkers for evaluating the responses to treatment and the status of ILD in anti-MDA5ab-positive DM. In this study, we further studied the importance of anti-MDA5ab levels and of ferritin and IL-18 concentrations in our patients. These biomarkers could be sometimes useful for evaluating ILD status and/or predicting the prognosis in patients with anti-MDA5ab-positive DM with several exceptional cases. A single-point evaluation of anti-MDA5ab levels and of ferritin and IL-18 concentrations has limitations in predicting the prognosis of ILD with DM. We consider that the timing of initial therapy and the anti-MDA5ab isotype, in addition to the patient's age, are also crucial factors for predicting the prognosis.