Respiratory syncytial virus (RSV) is one of the most prevalent agents of acute respiratory infections in children. Consistent data about seasonality of infections caused by this virus in Brazil proceed only from southern and southeastern regions. The purpose of this study was to determine the prevalence and the seasonal pattern of RSV infections in children living in Fortaleza city in northeastern Brazil. Nasopharyngeal aspirates of 1,950 patients attending in ambulatories, emergency room, and wards of Hospital Infantil Albert Sabin were collected and screened for RSV infections during 43 consecutive months. RSV was detected in 21% of the cases. RSV seasons in Fortaleza lasted 6 to 8 consecutive months, starting in January or February and finishing in July or August, and were associated with the rainy season.