The Porcine reproductive and respiratory syndrome virus (PRRSV) can persist for several weeks in infected pigs, and readily spreads from viraemic pigs to other pigs as well as to other herds. In regions with dense pig populations such as the North-Western part of Germany the virus is endemic. The aim of this study was to analyse the genetic diversity of PRRSV found in 18 herds in this region, which had pigs with respiratory or reproductive failures, and where PRRSV had been demonstrated by RT-PCR. Phylogenetic trees were calculated using the complete nucleotide sequences of the ORF5. Of the 65 samples sequenced, five PRRSV from four herds were of the North American (NA; Type 2) genotype, and 60 of the European (EU; Type 1) genotype. To ascertain if the field PRRSV varied with time and to monitor the health condition of the herds they were revisited 2 years later. Although only two herds still reported clinical signs, PRRSV was found by RT-PCR in 10 of the 18 herds. Phylogenetic analysis showed that of the 23 PRRSV sequenced, 15 were of the European (EU) genotype. The EU genotype isolates from both samplings could be assigned to one of 12 clusters. There was no indication for the existence of herd specific clusters. ORF5 sequence identities between PRRSV from one herd in one cluster were either 100%, or had single base exchanges. These data indicate that the mutation rates for the European field isolates are similar to that found for the NA genotype vaccine strain used in Germany.