An epidemiologic study was performed to determine the genetic variability of rotaviruses in Seoul, South Korea. In 3,174 stool specimens from children with acute diarrhea at five referral hospitals, 571 (18%) possessed the antigen of group A rotavirus detectable by ELISA - 10.8% in 2004 and 28.1% in 2005. VP7 genotyping revealed that the G3 type was found in 25.6% of all typed isolates, G4 in 23.8%, G2 in 21.6%, and G1 in 17.6%. VP4 genotyping showed that the P type was detected in 66.7%, P in 15.6%, P in 13.0%, and P in 0.2%. Because the variant P type could not be amplified initially by conventional P typing primers (1T-1), PCR were performed using newly designed 1T-1S primer, which revealed that 307 specimens were the variant P type. Uncommon combinations such as G4P and G2P were also found with relatively high prevalence, 14.6% and 12.8%, respectively. Variant P types were associated with an outbreak of rotavirus in 2005.