We report the pathotyping of six Australian isolates of Marek's disease virus-1 (MDV1) isolated between 1992 and 2004 and association of virulence with 'meq' gene polymorphism. Unvaccinated and herpes virus of turkeys (HVT)-vaccinated specific pathogen free chickens were challenged at day 5 with 500 plaque forming units of Marek's disease virus. The isolates induced gross Marek's disease lesions in 53 to 94%)A, of unvaccinated chickens, and HVT induced a protective index ranging from 38 to 100% by 56 days post challenge. This experiment provides evidence that current Australian isolates of MDVI vary significantly in pathogenicity. However, there was no clear evidence that the most virulent recent isolates were more pathogenic than isolates from the 1980s or that any of the isolates belong to the highest pathotype category of very virulent plus. Evidence is presented that virulence can be predicted by measurements taken as early as 13 days post challenge. The 'meq' gene sequences of five of the isolates used in the experiment were determined. When compared with the very virulent US isolate Md5, there was a 177 base-pair insertion and distinct point mutations in each of the five isolates. There were no individual mutations in the 'meq' sequences that correlated with levels of virulence. However, amino acid alignment of the five Australian and 14 international isolates revealed that the number of repeat sequences of four pro lines (PPPP repeats) in the 'meq' gene (overall range 2 to 8) was strongly associated with virulence across all isolates, with the most pathogenic isolates having the fewest number of repeats. The results suggest that the presence of the 177 base-pair insertion alone is not an indicator of attenuation. Rather, the number of PPPP repeats, independent of the presence of the insertion, is a better indicator of pathogenicity.