Background:Influenza-like illness (ILI) incidence estimates in individuals treated withimmunosuppressants and/or biologics and/or corticosteroid for an autoimmune orchronic inflammatory disease are scarce. We compared the ILI incidence amongimmunocompromised population and the general population.Method:We conducted a prospective cohort study during the 2017–2018 seasonalinfluenza epidemic, on theGrippeNet.frelectronic platform, which allows the collec-tion of epidemiological crowdsourced data on ILI, directly from the French generalpopulation. The immunocompromised population were adults treated with systemiccorticosteroids, immunosuppressants, and/or biologics for an autoimmune or chronicinflammatory disease, recruited directly onGrippeNet.frand also among patients ofthe departments of a single university hospital that were asked to incorporateGrippeNet.fr. The general population consisted of adults reporting none of the abovetreatments or diseases participating inGrippeNet.fr. The incidence of ILI was esti-mated on a weekly basis and compared between the immunocompromised popula-tion and the general population, during the seasonal influenza epidemic. Results:Among the 318 immunocompromised patients assessed for eligibility,177 were included. During the 2017–2018 seasonal influenza epidemic period,immunocompromised population had 1.59 (95% CI: 1.13–2.20) higher odds to expe-rience an ILI episode, compared to the general population (N=5358). An influenzavaccination was reported by 58% of the immunocompromised population, comparedto 41% of the general population (p< 0.001).Conclusion:During a seasonal influenza epidemic period, the incidence of influenza-like illness was higher in patients treated with immunosuppressants, biologics, and/orcorticosteroids for an autoimmune or chronic inflammatory disease, compared to thegeneral population.