Avian leukosis virus subgroup J (ALV-J) was isolated in meat-type breeder chickens for the first time in 1988 in the United Kingdom. Due to the application of an eradication program, there were fewer reports related to myelocytomatosis or ALV-J in China after 2013. However, there was another breakout almost simultaneously in six provinces of China in February 2018. On-site, 15- to 20-week-old broiler breeder chickens showed depression, paralysis and weight loss. Mortality for certain flocks reached 15%. Sick chickens showed numerous yellow-white neoplasms growing in the sternum, rib and lumbar vertebra and had hepatic and renal metastasis. Histopathological observation showed all neoplasms were myelocytomas, and there were massive myelocyte-like tumour cells in the liver, kidney and bone marrow. To explore the aetiology of this re-outbreak of myelocytomatosis in China, we collected tumour-bearing chickens and isolated six strains of ALV-J (GM0209-1 to -6). Phylogenetic analysis of gp85 and gp37 showed GM0209 strains were clearly distinct from the prototype strain of ADOL-7501, HPRS-103 and NX0101, and there was a mutation, R176G, in the conserved region between hr1 and hr2 regions of gp85, which was not found in other 44 ALV-J strains. The 3'UTR nucleotide sequences of GM0209 isolates showed there was a signature deletion of 11 nt that was also present in 3'UTR sequences of SCDY1 and NHH, two isolates that have a reported association with haemangioma, indicating this deletion could not determine the tumour type induced by ALV-J. Although the eradication program of ALV-J has been successfully applied in China, the outbreak of ALV-J still occurred, and the virus strain spread quickly. Thus, the biocharacteristics and pathogenesis of mutational ALV-J should be further studied.