Interleukin 17A (IL-17A) is implicated in the pathogenesis of several neuroimmunologic diseases. We aimed to evaluate the relationship between IL-17A and seizure severity in patients with epilepsy. Seventy patients with probable symptomatic epilepsy and 68 healthy controls were included. Interictal serum IL-17A and related cytokine (IL-23, IL-6, IL-1β, interferon gamma (IFN-γ), and IL-10) levels were measured. The relationship between seizure severity and cytokine concentrations was assessed by Spearman correlation and multivariate linear regression test. IL-17A levels in the cerebrospinal fluid (CSF) were tested in 30 additional patients with epilepsy, either in the postictal or interictal period and 15 patients with idiopathic inflammatory demyelinating diseases (IIDDs). Interictal serum IL-17A levels were significantly elevated in patients with epilepsy compared to controls. IL-6, IFN-γ, and IL-1β levels were also markedly elevated. Spearman correlation analysis revealed positive correlation between IL-17A, IL-6 levels and Veterans Administration Seizures Frequency and Severity Rating Scale score(VA score); IFN-γ, IL-10 levels, and National Hospital Seizure Severity Scale (NHS3) score. In addition, IL-17A levels correlated significantly with seizure frequency. Multivariate linear regression test showed that only IL-17A levels were independently positively correlated with VA scores (B = 0.288, p = 0.027). Postictal IL-17A levels in the CSF were significantly elevated compared to interictal patients and patients with IIDDs. Our results suggest that interictal IL-17A levels correlated highly with seizure severity.