From January 1998 to July 2000, 2,456 clinical samples, including lung, tonsil, lymph node, and serum, from 760 cases submitted to the Animal Health Laboratory, Ontario, Canada, were tested for porcine reproductive and respiratory syndrome viruses (PRRSV) using reverse transcriptase polymerase chain reaction (RT-PCR) and RT-PCR product restriction fragment length polymorphism (RFLP) analysis. A total of 516 samples from 284 cases were RT-PCR positive for the PRRSV open reading frame (ORF) 7 sequence. The RT-PCR RFLP typing assay was performed using 2 different sets of primers, amplifying 716 or 933 base pairs of ORF 4, 5, and 6 of PRRSV. Samples from 254 cases were typeable, yielding 34 different RFLP types. Of these, 164 cases had 32 different RFLP types of field or intermediate strains, 86 had a pattern similar to a commercial PRRSV vaccine or VR 2332 strain of the virus, 4 had a RFLP type shared by another commercial vaccine and a field strain. In 4 cases, 2 different RFLP types were identified from tissues from different pigs that were submitted at the same time from the same farm. Of the 195 farms that submitted PRRSV PCR-positive samples, 48 submitted samples on more than 1 occasion during the specified time frame. In 23 of those 48 farms, RFLP patterns of PRRSV differed between submissions, whereas in the other 25 farms, the RFLP pattern remained unchanged. There were 34 different PRRSV patterns identified from 236 cases using the primer set amplifying 716 base pairs of PRRSV. There were 18 cases, consisting of 9 different patterns, typeable only by using the primers amplifying a 933-base pair fragment of the virus.