OBJECTIVE We evaluated the severity of influenza A(H1N1)v clinical forms among infants less than 6 months of age. This population group was considered a high-risk group, so all people around them should be vaccinated first. METHODS In south-western France in Aquitaine, we collected all infants less than 6 months of age during a period between the 6th September 2009 and the 6th January 2010 with influenza A(H1N1)v confirmed by PCR. For each of them, the risk factors, clinical presentation, hospitalization, and course of, the disease were identified. We compared two groups: children under 3 months and infants aged 3-6 months. RESULTS We identified 74 infants. The average age was 3 months. Sixteen infants had at least 1 risk factor: 9 respiratory diseases (12%), 8 born prematurely (but there was no preterm baby under 33 weeks); one infant presented a cardiac disease, and another 1 epilepsy. Five infants showed no fever, 73% had cough, and 24% had gastro-intestinal symptoms. Infants under 3 months of age presented less cough (P<0.025) and fewer gastro-intestinal symptoms (P<0.01) than older ones. Only 5 infants needed oxygen and 4 presented pneumonia. Forty-eight infants were hospitalized, including 1 in intensive care, with a median duration of 3 days. Forty-five percent spent 2 days or less in the hospital. Infants under 3 months of age were more often hospitalized (P<0.001). CONCLUSIONS Infants under 6 months of age did not present a severe form of influenza A(H1N1)v. Infants under 3 months of age were less symptomatic than older infants and were often hospitalized, but hospital stays were short with a good outcome.