OBJECTIVE To evaluate the prevalence and seasonality of human respiratory syncytial virus (HRSV) in children aged 0 to 6 years, hospitalized with acute lower respiratory infection (ALRI) in São José do Rio Preto, SP, Brazil, and the association between age, diagnosis, and HRSV. METHODS Between May 2004 and September 2005, we studied 290 consecutive episodes of community-acquired ALRI in children aged 0 to 6 years admitted to the Hospital de Base of São José do Rio Preto. In order to detect HRSV, nasopharyngeal secretion samples were collected and RT-PCR molecular analysis was performed. RESULTS The HRSV prevalence was 29.3% for the cases of hospitalized patients with ALRI. ALRI was common in infants (median age = 13.5 months). HRSV was more frequent in cases of bronchiolitis (64%) and during the first year of life (35%). Episodes of HRSV infection occurred between fall and spring, showing higher frequency in 2004 than in 2005. Clinical and radiological criteria were not sufficient to establish the diagnosis of infection with HRSV. Antibiotic therapy was used in 78.8% of episodes of HRSV. CONCLUSIONS There was a high prevalence of HRSV in children aged 0 to 6 years who were hospitalized for ALRI, predominantly in younger patients or those with bronchiolitis. The circulation of the virus varied in the two years studied. Our results suggest the need for laboratory diagnosis of HRSV in the clinical practice.