OBJECTIVE To explore the role of interleukin-17 (IL-17) in the evolution of viral myocarditis (VMC) into dilated cardiomyopathy (DCM). METHODS A mouse model of VMC was established in 100 male Balb/c mice by intraperitoneal injection of coxsackievirus B3. The expression of IL-17 protein in the cardiac tissue of the mice was detected immunohistochemically, and IL-17 mRNA in the splenocytes was examined by reverse transcription-polymerase chain reaction (RT-PCR). IL-17 levels in the plasma, peripheral blood mononuclear cell (PBMC) culture supernatants, and phytohemagglutinin (PHA)-stimulated PBMC culture supernatants were measured in 30 DCM patients, 26 non-DCM patients and 20 normal adults using enzyme-linked immunosorbent assay (ELISA), and IL-17 mRNA expression in the PBMCs was detected using RT-PCR. RESULTS The levels of IL-17 mRNA in the splenocytes of the mice with VMC were significantly higher at 4 and 6 weeks than those at 8 weeks (P<0.01), but not detected at 2 weeks. No IL-17 expression was found in the ventricular tissue of the mice at 2 weeks, but peaked at 4 weeks followed by gradual decrease (P<0.01). IL-17 level in PHA-stimulated PBMC culture supernatants but not the plasma, and its mRNA level in PHA-stimulated PBMCs but not the PBMC culture supernatants, were significantly elevated in DCM patients as compared with those in non-DCM patients and normal control subjects. CONCLUSIONS The mouse model of VMC in the chronic phase and DCM patients express high levels of IL-17, which may contribute to the transition from VMC to DCM.