Influenza epidemics occur worldwide annually. The incidence of influenza shows a seasonal pattern in temperate areas, but little is known about influenza seasonality in tropical regions. The objective of this study was to determine the prevalence and the seasonal pattern of influenza infections in children living in the city of Fortaleza in northeastern Brazil. An indirect immunofluorescence assay was performed on nasopharyngeal aspirates collected from children attending in ambulatories, emergency rooms, and wards of the Hospital Infantil Albert Sabin with suspicion of acute respiratory infection during 7 consecutive years (2001–2007). Influenza viruses were detected in 6.3% (234/3,708) of specimens. Laboratory-based surveillance data showed a clear annual epidemic cycle of influenza, with a peak usually occurring in the rainy periods. In Fortaleza, flu infections occurred at a low level throughout the year but exhibit a marked seasonal increase during the rainy season.