The avian leukosis viruses (ALVs) are a major group of retroviruses associated with neoplastic diseases in poultry. The ALV-J strain was identified as a cause of myelocytomas in broiler breeder and broiler chickens in the UK in the 1980s; however, following eradication of the virus,commercial broilers have remained free of infection since the early 2000s. A pet chicken was submitted to Animal and Plant Health Agency (APHA) in 2013 with a history of croaking respirations, abnormality of the left eye and apparent paralysis. Postmortem examination of the bird showed widespread tumour-like infiltration of many organs, including the pectoral muscles, internal organs, sternum and ribs. Histopathological examination of the affected tissues revealed myelocytoma formation typical of the lesions associated with ALV-J, and the virus was confirmed by PCR testing and sequencing. Virus was not detected in blood samples in the other five chickens remaining in the flock. The source of infection was not established. This was the first time ALV-J had been seen in the UK since its eradicationand the case highlights the importance of continued surveillance of backyard and hobby chickens to detect potential new and re-emerging disease threats, such as ALV-J, which may be of significance to the wider poultry population.