A naturally occurring lymphoproliferative disease was found in 3 flocks of Japanese quail in Mexico. The tumorlike lesions were detected mainly in livers and spleens. Histopathology revealed a pleomorphic lymphoproliferative infiltration with dark-staining large cells, lymphocytes of different sizes, and reticulumlike cells. Marek's disease (MD) herpesvirus was not isolated from 74 quail tested, nor were antibodies to MD detected in 84 sera. Antibodies to reticuloendotheliosis (3/24) and avain leukosis virus subgroup A (2/24) were present. The possible etiology of this disease is discussed.