BACKGROUND: Infants and toddlers have the highest influenza hospitalization rate in pediatrics. Although the impact of this virus in children has been recognized, there is no defined statement related to vaccination in this population. AIM: To describe clinical and epidemiological characteristics of complicated influenza infections in hospitalized children. MATERIAL AND METHODS: All hospitalizations due to influenza virus were recorded prospectively between March and June 2004. RESULTS: We registered 40 laboratory-confirmed influenza admissions. Median age was 24 months (range: 15 days-14.5 years), 52% males, 18 younger than 2 years. Most of them had an underlying medical condition. The most common conditions were recurrent wheeze in 17, a neurological disease in 7 and asthma in 6. Twenty had more than one condition and 15 were previously healthy. The average days of respiratory symptoms and fever prior to admission were 5 and 3, respectively. The most common discharge diagnoses were concomitant viral-bacterial pneumonia in 53%, viral pneumonia in 38% and laryngitis in 8%. Influenza virus A was identified in 34/40 children. Oxygen supplementation was required by 34 cases; 20% of which required an O2 inspired fraction over 40%. The average days of hospitalization and oxygen were 4 and 3, respectively. Eleven children were treated with amantadine and 21 with antimicrobials. Four children were admitted to pediatric intensive care units and two cases required non-invasive ventilatory support. No deaths were recorded. CONCLUSIONS: Our data confirms the importance of influenza virus infection in children, as measured by admission rates, complications and length of hospital stay. Young children are a risk group for which immunization is recognized as protective.