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Diagnosing influenza at an early stage of illness is important for the initiation of effective antiviral treatment. However, especially in young children, influenza often commences with an abrupt onset of fever, with full-blown respiratory symptoms developing only later. We determined the feasibility of diagnosing influenza in young children already during the first signs of the illness. During confirmed influenza activity, we obtained nasal swabs from children aged 1-3 years who presented as outpatients within 24 hours of the onset of fever (≥38.0°C). The specimens were tested for influenza viruses with viral culture, antigen detection, PCR, and a rapid point-of-care test (Actim Influenza A&B, Medix Biochemica, Finland). In addition, follow-up specimens were obtained from a proportion of children 3-7 days later. Influenza virus was detected already within 24 hours of symptom onset in 56 of 61 (92%; 95% CI 82-97%) children in whom influenza was eventually confirmed in the laboratory. A total of 158 rapid tests performed within 24 hours of symptom onset yielded a sensitivity of 90% (95% CI 74-98%) for influenza A viruses but only 25% (95% CI 3-61%) for influenza B viruses (P < 0.001), resulting in an overall sensitivity of 77% (95% CI 61-89%) and specificity of 99% (95% CI 95-100%) for all influenza viruses. In most young children, influenza can already be accurately diagnosed within 24 hours of symptom onset. The rapid point-of-care test used was sensitive and specific for diagnosing influenza A, but its sensitivity for influenza B was limited.