Norovirus is one of the most common causes of nonbacterial gastroenteritis in humans. A total of 603 fecal specimens collected from sporadic pediatric cases of acute gastroenteritis in Japan from 2004 to 2005 were tested for the presence of norovirus by RT-PCR. It was found that 51 (8.5%) specimens were positive for norovirus. The norovirus genotypes detected in this study were GII/1, GII/2, GII/3, GII/4, GII/6, and GII/7. Of these, GII/3 was the most predominant (52.9%), followed by GII/4 (37.2%) and others. It was noticed that four distinct types of recombinant noroviruses were co-circulating and the variant norovirus GIIb suddenly emerged to be the leading strain in Japan for the first time. A novel norovirus nomenclature was proposed, in which worldwide noroviruses were classified into seven distinct genogroups (I-VII). Norovirus GI and GII consisted of 16 genotypes with 32 subgenotypes and 23 genotypes with 34 subgenotypes, respectively. Of note, human and porcine noroviruses had a close genetic relationship within GII. Interestingly, multiple short amino acid motifs located at N terminus, S domain, P1 domain, P2 domain, and C terminus of capsid gene correctly defined the phylogenetic norovirus genogroups, genotypes, and subgenotypes. Another interesting feature of the study was the identification of eight hitherto unreported recombinant noroviruses. It was noteworthy that three different types (intergenogroup, intergenotype, and intersubgenotype) of recombination in noroviruses were also found. This is the first report to demonstrate the existence of intergenogroup and intersubgenotype recombinations in noroviruses and highlights a possible route of zoonoses in humans because porcine, bovine and murine noroviruses belong to genogroups II, III, and V, respectively.