Two distinct genotypes of PRRSV, European and North American (NA), are described with each genotype independently evoluting in each continent. In several countries, however, both genotypes concurrently exist in which may play an important role in increased severity of clinical disease and genetic modification of PRRSV. The objectives of the study were to investigate pneumonic lesion and ongoing changes in open reading frame 5 of PRRSV in pigs simultaneously infected with both EU and NA PRRSV isolates. Twenty nine, PRRSV free, pigs were randomly allocated into 4 groups; NEG, US, EU and MIX. NEG was no-challenge control. Groups US and EU were intranasally challenged with NA and EU PRRSV isolate, respectively. Group MIX was intranasally challenged with both isolates. 5 and 3 pigs per group were sacrificed on 10 and 30 days post infection (DPI), respectively. Lungs were assessed for PRRSV-induced macroscopic pneumonic lesion. PRRSV was isolated from bronchoalveolar lavage fluid (BALF). At 10 DPI, PRRSV-induced macroscopic pneumonic lesions were significantly more severe in pigs concurrently infected with both EU and NA isolates than pigs infected with either isolate. At 30 DPI, there was no difference between single and dual infection. Several plaque-cloned viruses were recovered and compared to the parent virus by sequencing of open reading frames (ORF) 5 of the genome. EU or US related progeny viruses were isolated from either EU or US infected pigs. In dual infected pigs, however, 3 clades of progeny viruses were determined; EU- and US-related, and a group distinct from either EU or US. The sequence analysis showed the changes in ORF 5 occurred more rapidly in pigs concurrently infected with both EU and NA isolates than pigs infected with either isolate. Progeny viruses isolated at 30 DPI were closely related to the parent virus than progeny viruses isolated at 10 DPI by which were genetically more diverse. Interestingly, recombination between the 2 genotypes was observed at every time points. The findings suggest the presence of 2 distinctive PRRSV genotypes can accelerate the change of viral genome and increase the severity of the disease. Each genotype independently evoluting and the recombination between the 2 distinct genotypes can occur, but its ability to persistently infect remained to be further investigated.