C1q is an important component of the classical complement pathway. This study aimed to investigate C1q concentrations in the sera of PM/DM patients. C1q concentrations were measured in the sera of 87 PM/DM patients and 100 healthy subjects. In addition, the association between C1q concentration and laboratory indexes such as CK, AST, ALT, LDH, hsCRP, and ESR were also evaluated. Our data indicated no significant difference in the serum C1q concentration between PM/DM patients and healthy subjects. However, serum C1q concentrations were observed to be significantly higher in PM/DM patients in patients group with elevated ESR than in those with normal ESR (207.55±37.54 mg/L and 180.69±38.90 mg/L, respectively; P=0.008). Furthermore, there was a positive correlation between serum C1q concentration and ESR (P=0.002), but no association was observed with CK, AST, ALT, LDH, and hsCRP levels. This study demonstrated C1q levels were elevated in PM/DM patients with higher ESR and revealed significant positive correlation. However, the exact role of the classical complement pathway in PM/DM needs further elucidation. © 2019 by the Association of Clinical Scientists, Inc.