Akabane disease, an infectious disorder causing congenital abnormalities in calves, was studied in naturally affected calves between 1972 and 1974 in Japan. The animals included one three-month fetus from which a strain of Akabane virus (OBE-1) was isolated, and a total of 177 stillborn or premature fetuses and deformed or infirm newborn calves that died within a few days of birth. The three-month fetus had nonpurulent encephalomyelitis in the undifferentiated central nervous system, characterized by necrosis of the nerve tissue and endothelial proliferation in the encephalitic process; and polymyositis in the myotubule phase with parenchymal degeneration and cellular infiltrates in the skeletal muscle. The full-term fetuses and newborn calves had nonpurulent encephalomyelitis in the early stage of the epizootic. In the middle to late stages, a dysplastic muscular change which we call runt-muscle disease was seen simultaneously with a decrease in number of ventral horn neurons in the spinal cord and arthrogryposis in the legs. Cystic cavities and thick vascular walls sometimes were seen in the central nervous system in these stages. Hydranencephaly was prominent in the late stage.