Pandemic influenza A (H1N1) virus was first reported in April 2009. The aim of this study is to describe the clinical course of patients with influenza-like illness treated in a tertiary care pediatric hospital. Cross-sectional analytical study, encompassing the period from April 2009 to March 2010. Clinical and demographic information was obtained from clinical records. Data analysis was carried out using descriptive statistics, using a univariate analysis with the chi-square test, the exact Fisher test, and the Mann-Whitney U test for quantitative variables. 240 patients were included, out of which 53.9 % were female; median age was 5 years. Sixty four cases (26.6 %) were confirmed, 38 % had and underlying condition, and 10 % had received the influenza vaccine. One hundred and sixteen patients (48 %) were hospitalized. With regard to mortality, 10 out of 64 confirmed cases died, 3 of the 86 of the disregarded cases, and 2 of 90 without a confirmatory test died (p < 0.05). The patients who died started antiviral treatment on day 7; conversely, those who survived started the treatment on day 4 (p < 0.05). Lethality was higher in patients with confirmed infection. Antiviral treatment within the first 48 hours was observed to be essential for patients with risk for the development of complications.