Objectives: This study aimed to examine the diagnostic and prognostic value of the platelet-to-lymphocyte ratio (PLR) and neutrophil-to-lymphocyte ratio (NLR) in patients with polymyositis/dermatomyositis (PM/DM).Method: Clinical data of 200 patients with PM/DM and 204 healthy controls were retrospectively reviewed. We examined whether the PLR and NLR were associated with PM/DM.Results: The PLR and NLR were higher in patients with PM/DM than in controls (both P < 0.001). The PLR and NLR were higher in patients with DM than in those with PM (both P < 0.01). The PLR was higher in the anti-melanoma differentiation-associated protein-5 (anti-MDA5) + PM/DM group than in the anti-MDA5- PM/DM group (P = 0.002). The NLR in non-survivors was higher than that in survivors (P = 0.01). The NLR was positively correlated with the occurrence of interstitial lung disease (ILD). The PLR and NLR were independent predictors of PM/DM, as well as risk factors (both P < 0.001). Moreover, the NLR had a predictive value for PM/DM-ILD and was closely related to mortality (P = 0.033, P = 0.003, respectively).Conclusions: Patients with PM/DM have a higher NLR and PLR than healthy controls, especially in those with anti-MDA5+. The PLR and NLR are independent risk factors for PM/DM and have some predictive value. The NLR is correlated with ILD and associated with an increased risk of mortality in patients with PM/DM. The NLR and PLR may be simple, economical, and accurate diagnostic and prognostic markers for patients with PM/DM.[Table Removed]