Epidemiological studies of subgroup J avian leukosis virus (ALV-J) and reticuloendotheliosis virus (REV) infections were conducted during 1999 to 2009 on 29 chicken flocks of various commercial and local breeds located in six provinces in China. Samples were typically from chickens with myelocytomas or proventricular lesions. ALV-J was isolated from 25 flocks including seven out of seven flocks containing "yellow chickens" or other local breeds and several flocks of layer chickens. REV was isolated from 19 flocks. Co-infection of ALV-J and REV was confirmed in 11/38 birds of 8/15 flocks with myelocytomatosis and in 11/24 birds of 3/5 flocks with proventricular lesions. Both ALV-J and REV were vertically transmitted in four breeder flocks of local chicken breeds. One experiment in specific pathogen free chickens indicated that co-infection at hatch strongly depressed antibody responses to ALV-J but not to REV. The results suggest that infections with both ALV-J and REV have become endemic in local breeds, including breeder flocks where both viruses appear to be perpetuated by vertical transmission. In addition, both viruses were present in at least some commercial broiler, layer and breeder flocks. Eradication programmes are needed but will be difficult, especially in local breeds. This unprecedented concurrence of simultaneous endemic infections with ALV-J and REV may have serious economic consequences and needs to be better understood.