Santé publique France has been monitoring severe cases of influenza in intensive care units since 2009. A case of severe influenza is defined as any patient hospitalized in an intensive care unit who presents with: a biologically confirmed diagnosis of influenza or a severe clinical form with no other identified etiology, whose clinical presentation and medical history suggest a diagnosis of influenza, even if biological confirmation cannot be obtained (probable case). All cases must be reported to the regional units of Santé publique France. The objective of the study was to analyze the completeness of this surveillance and to estimate the number of severe influenza cases admitted to intensive care units in mainland France for the 2017–2018 season.