Human coronavirus NL63 (HCoV-NL63) has been found in children presenting with respiratory tract infections (RTIs). However, the epidemiology and clinical course of this newly identified virus have not been fully elucidated. This study investigated the epidemiology, seasonality, and clinical features of HCoV-NL63 in Hong Kong children. This study consisted of two cohorts of children hospitalized in a university-affiliated teaching hospital. In the 12-month retrospective part of the study, reverse transcription-PCR was used to detect HCoVs in nasopharyngeal aspirates (NPAs). Positive samples were sequenced to confirm the identity of the virus and to determine its phylogenetic relationship with the HCoV-NL63 strains found elsewhere. The second part covered a subsequent 12-month period in which patients were prospectively recruited. Altogether, 1,981 and 1,001 NPA specimens were studied in 2005-2006 and 2006-2007, respectively. Seventy-four (2.5%) HCoV isolates were identified and consisted of 17 (0.6%) HCoV-NL63 isolates, 37 (1.2%) HCoV-OC43 isolates, 14 (0.5%) HCoV-HKU1 isolates, and 6 (0.2%) HCoV-229E isolates. HCoV-NL63 infection was more common in 2006-2007 than 2005-2006 (1.2% and 0.3%, respectively; P = 0.006). From 2005 to 2007, the peak season for HCoV-NL63 infection was in September-October, which was earlier than the peak for HCoV-OC43 infections (December-January). HCoV-NL63-infected patients were younger and more likely to have croup, febrile convulsions, and acute gastroenteritis. The majority of local HCoV-NL63 isolates were phylogenetically closely related to those found in Belgium and The Netherlands. In conclusion, HCoV-NL63 is an important yet uncommon virus among our hospitalized children with acute RTIs.