BACKGROUND:: To determine the rates of influenza-related hospitalization and to know the clinical manifestations and underlying diseases in children younger than 3 years who are hospitalized with influenza. METHODS:: Retrospective, descriptive study (1996-2003), performed in a tertiary teaching hospital in Madrid. Data of hospitalized children, younger than 3 years, with influenza virus isolation from nasal aspirates were collected. Rates of hospitalization for every year were calculated. RESULTS:: Overall, 146 children hospitalized with influenza were identified: 117 had community-acquired influenza as the only disease, 18 had community-acquired influenza and were coinfected with other pathogens, and 11 had nosocomial infection. Rates of influenza hospitalization for years 1996, 1997, 1998, 1999, 2000, 2001, 2002, and 2003 were 0.42, 0.11, 1.46, 1.54, 0.53, 0.25, 0.19, and 0.82, respectively, per 1000 children younger than 3 years of age. Children </=1 year of age accounted for almost two thirds of admissions.Bronchitis/bronchiolitis (42 children), pneumonia (11 children), fever without source (36 children), and suspected sepsis (9 children) accounted for almost 90% of all hospitalizations in children with community-acquired influenza.Forty-seven patients (40%) had underlying diseases, mainly chronic pulmonary disease and congenital heart disease. Ten patients (8.5%) with community-acquired influenza A and underlying conditions were admitted to the intensive care unit. CONCLUSIONS:: Influenza is an important cause of hospitalization in young children. The use of influenza vaccine in high-risk children could prevent hospitalizations and cases of influenza-related diseases.