The clinical and neuropathological findings of spinal muscular atrophy (SMA) in Holstein-Friesian calves are described in four females and one male from a dairy farm composed of 150 cows and 2 breeding bulls. Locomotion difficulties started at the age of 15 days, and progressed to paraparesis and tetraparesis in 2 weeks. Signs consistent with denervation were revealed with electromyography. The neuropathological examination showed degeneration and loss of motor neurons in the spinal cord, together with astrocytosis. Among the remaining motor neurons were ghost cells and neurons filled with accumulations of straight filaments measuring 10-12 nm in diameter, which were strongly immunoreactive with antibodies produced against phosphorylated neurofilaments. Degenerating cells in SMA did not stain with the method of in situ labelling of nuclear DNA fragmentation and did not show c-Jun immunoreactivity. This feature contrasts with the in situ labelling of DNA breaks of apoptotic cells and with the strong c-Jun immunoreactivity restricted to dying cells during the whole process of naturally occurring cell death in the developing central nervous system. These features suggest that cell death in SMA differs from programmed cell death during normal development, and that pathological cell death in SMA should not be considered as a mere persistence or reactivation of normally occurring developmental cell death.