BackgroundThe role of cytokines in the pathogenesis of febrile seizures (FSs) is unclear, and information regarding cytokine production outside of FS episodes is scarce.MethodsIn our controlled follow-up study of patients with FSs, we compared the levels of 12 serum cytokines after the patients’ first FSs, during febrile episodes without FSs, after recurrent FSs, during healthy periods after FSs, and between patients and controls.ResultsTwo-hundred fifty-one patients with first FS participated in the study, of whom 17 (mean age 1.6 years, SD 0.7) with recurrent FSs completed the protocol as required by the sample size calculations. The mean IL-1RA level was higher after the first FSs (2580 pg/mL, SD 1516) than during febrile episodes without FSs (1336 pg/mL, SD 1364,P= 0.006) and healthy periods after FSs (474 pg/mL, SD 901,P= 0.001). IL-1RA levels were also higher during first (2580 pg/mL) and recurrent FSs (2666 pg/mL, SD 1747) in comparison with febrile controls (746 pg/mL, SD 551) (P< 0.001 andP= 0.001, respectively), but there was no difference in the IL-1RA between febrile episodes without FSs and febrile controls.ConclusionsPatients with FSs produce stronger inflammatory reactions during febrile episodes with FSs compared with febrile episodes without FSs and febrile controls.ImpactIn patients with FSs, IL-1RA was higher following first FS than during febrile episodes without FSs and healthy periods after FSs.IL-1RA was higher in patients with FSs following first and recurrent FSs than in febrile controls.There was no significant difference in IL-1RA between febrile episodes of patients without FSs and febrile controls.Using IL-1RA as a surrogate marker of IL-1 axis activity, our results indicate that patients with FSs produced stronger inflammatory reactions during FS episodes but not during other febrile episodes or healthy periods after FSs.Cytokines may play a role in pathogenesis of FSs.