From 1991 to 1994, 143 polioviruses were isolated from patients with acute flaccid paralysis in northern Vietnam. Of these 143 isolates, 133 were type 1 and five of each were type 2 and type 3. These isolates were intratypically differentiated by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Of the 133 type 1 isolates, 113 were wild strains and only 20 isolates were of Sabin vaccine-like strains. These 113 isolates were divided into seven groups by PCR-RFLP patterns and were also classified into three genomic groups by nucleotide sequencing and phylogenetic analysis. One of the three genomic groups accounted for the majority (75) of these isolates. The isolates belonging to the predominant group were found during 1991-1993. The second group, which included 17 isolates, were detected only in 1993. These isolates had genome sequence similar to isolates in southern Vietnam in the same year. The third group of isolates were detected only in 1991 and were considered to be Mahoney-like wild strains. All isolates examined were different from those obtained in other countries. In 1994, however, no wild-type polioviruses were isolated in our study. These results reveal that three unique strains were circulating in northern Vietnam in recent years, and indicate that the incidence of poliomyelitis due to wild poliovirus is decreasing.