AimsThe aim of this study was to investigate the common occurrence of reticuloendotheliosis virus (REV) among Gallid herpesvirus 2 (GaHV-2) infected chickens. The possible cause of this co-occurrence may be linked to contaminated vaccine stocks, which were also examined. Methods and ResultsThe study was conducted on 25 field isolates of GaHV-2 collected between 2007 and 2013 from vaccinated chickens. Additionally, 10 commercial Marek's Disease vaccine stocks manufactured between 1993 and 2013, comprising of FC126 HVT, CVI988/Rispens and bivalent HVT+Rispens vaccines were examined. Chicken isolates were collected from the liver. Due to difficulties in differentiation between GaHV-2 and REV, by observation of clinical signs or lesions presented in liver or spleen, loop-mediated isothermal amplification (LAMP and RT-LAMP) as well as PCR-based methods were applied. ConclusionsThe co-occurrence of GaHV-2 and REV genetic material was shown in 24 (96%) of 25 examined isolates. A marginal REV contamination was detected in three out 10 (30%) commercial vaccine stocks, mainly in bivalent HVT+Rispens vaccines produced between 2009 and 2012. Significance and Impact of the StudyOur results indicated the common occurrence of GaHV-2 and REV in Polish chicken flocks, which is probably linked to contaminated HVT+Rispens vaccine stocks. Reasons for the detection of a marginal REV contamination need to be further elucidated.