- Objectives To describe the clinical characteristics of children hospitalized with 2009 influenza A(H1N1) infection in Israel and the risk factors associated with this infection. Design Prospective collection of data on children hospitalized with 2009 influenza A(H1N1) infection. Setting Seven medical centers around Israel. Patients From July 12, 2009, to December 24, 2009, all patients 18 years or younger hospitalized with acute respiratory or acute unspecified febrile illness were screened for 2009 influenza A(H1N1) virus by reverse transcription–polymerase chain reaction. Intervention Prospective data collection for patients with confirmed infection. Main Outcome Measures Clinical characteristics of patients and hospitalization rates. Results The mean age of 478 patients studied was 6.1 years. Forty-two patients (8.8%) were admitted to the pediatric intensive care unit; 3 patients (0.6%) died. The most frequent clinical presentations were pneumonia, influenza-like illness, wheezing exacerbation, and convulsions. Predisposing underlying illnesses were detected in 48.7% of patients. Patients with metabolic and neurologic disorders were at highest risk for severe complications (relative risk, 6.5 and 2.9, respectively). In addition, patients with cyanotic heart lesions and infants 3 months or younger who were born at 33 weeks' gestation or earlier tended to require higher rates of mechanical ventilation. The hospitalization rate for 2009 influenza A(H1N1) was 0.7 per 1000 children. The mortality rate was 3.6 per 1 000 000 children. Conclusions The severity variables for 2009 influenza A(H1N1) were similar to the figures reported for seasonal influenza. Patients with underlying metabolic and neurologic metabolic disorders and presumably patients with cyanotic heart lesions and infants born prematurely are at highest risk for severe complications following 2009 influenza A(H1N1) infection.