On occasion, outbreaks of infection with adenovirus types 3, 7, and 21 cause severe lower respiratory tract infections (LRTIs) in children. From 1990 to 1998, all cases of LRTI due to adenovirus at the Seoul National University Children's Hospital, Seoul, Korea, were reviewed. Adenoviruses were recovered from nasal aspirate specimens of 87 (5.9%) of 1472 children with LRTI. The principal adenovirus serotypes were type 2 (13 [15%] of 87 strains), type 3 (13 [15%]), and type 7 (36 [41%]). Of the 87 infections, 62 (71%) occurred in children <2 years of age, and 81 (94%) occurred in children <5 years of age. Infections due to types 3 and 7 occurred during epidemics, whereas infections due to type 2 occurred sporadically. For patients who were infected with types 3 and 7, extrapulmonary abnormalities were more common and homogeneous consolidation and pleural effusion were frequently identified on radiographs. The mortality rate was 12% overall and 19% among patients who were infected with type 7. Residual sequelae were identified in 6 (50%) of 12 patients who were infected with type 3 and in 9 (25%) of 36 who were infected with type 7. The data confirm that adenovirus types 3 and 7 can cause epidemics of severe LRTI in young children. Epidemics of LRTIs caused by adenovirus types 3 and 7 in Korea have not been described in reports published elsewhere.