Objective To explore the correlation between neutrophil-lymphocyte ratio (NLR) and diagnosis and disease activity of dermatomyositis (DM). Methods The clinical and laboratory data of 50 DM patients, who were admitted for the first time in Changzheng Hospital or Changhai Hospital of Naval Medical University (Second Military Medical University) from Jan. 2016 to Dec. 2019, were retrospectively analyzed. During the same period, 31 healthy controls, 26 rheumatoid arthritis (RA) patients and 27 ankylosing spondylitis (AS) patients were included in control groups. Mann-Whitney U test was used to compare the NLR values between DM, healthy control, RA and AS groups. Spearman rank correlation analysis was performed to explore the relationship of NLR with creatine kinase (CK), lactic dehydrogenase (LDH), aspartate aminotransferase (AST), alanine aminotransferase (ALT), C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR). Receiver operating characteristic (ROC) curve was drawn to evaluate the diagnostic efficiency of NLR for DM, and the area under ROC curve was compared using Z test. Pairwise Wilcoxon signed rank test was carried out to compare the NLR, CK, AST, ALT and LDH of DM patients before and after treatment. Results The NLR values were significantly higher in the DM, RA and AS groups than that in the healthy control group (U=29.0, 156.0, and 194.0, all P<0.01). The NLR value was significantly higher in the DM group than those in the RA and AS groups (U=333.0 and 213.0, both P<0.01). Spearman rank correlation analysis showed that NLR was positively correlated with CK, LDH, AST, ALT, CRP and ESR in DM patients (r_s0.540, 0.288, 0.440, 0.498, 0.304, and 0.311, all P<0.05). The areas under the ROC curve of NLR, LDH, CK, ALT and AST in the diagnosis of DM were 0.981, 0.954, 0.931, 0.913 and 0.867, respectively, and the best cut-off values were 2.824, 241.0 U/L, 101.0 U/L, 25.50 U/L and 30.00 U/L, respectively. The area under the ROC curve of NLR was significantly higher than those of ALT and AST (Z=2.05 and 2.67, P=0.040 and 0.008). There was no significant difference in the area under the ROC curve among the other laboratory indicators (P>0.05). The NLR, CK, AST, ALT and LDH of DM patients were significantly decreased after treatment versus before treatment (Z=3.28, 4.68, 4.62, 2.12, and 4.44, all P<0.05). Conclusion NLR can be used to evaluate the disease activity and severity of DM, and it is helpful for the clinical diagnosis and prediction of treatment response in DM.