SUMMARY The 1974 epizootic of congenital bovine arthrogryposis/hydranencephaly syndrome in south‐eastern New South Wales afforded the opportunity to study the pathology, serology and virology of field cases from the outbreak. For ease of description, the lesions observed were grouped into 5 stages which possibly approximate to the gestational age of the foetus at the time of initial insult Group 1: calves born incoordinate and microscopic examination revealed a universal mild to moderate non‐suppurative acute encephalomyelitis. Group 2: calves born incoordinate or showed mild arthrogryposis and microscopically a mild to moderately severe acute Wallerian type degeneration of all zones of the spinal cord except the dorsal funiculi was observed. Group 3: calves born with arthrogryposis and microscopically there was a severe diffuse loss of myelinated fibres in the lateral and ventral funiculi of affected areas of the spinal cord. There was also a moderate to severe atrophy of the skeletal musculature. Group 4: calves born with hydranencephaly, sometimes associated with arthrogryposis, with almost complete replacement of both cerebral hemispheres by a fluid filled cavity. Group 5: calves born with micrencephaly and hydranencephaly and sometimes arthrogryposis and loss of anterior and mid brain stem and cerebellar cavitation. There was often an overlap of the lesions between groups and the groups have been placed in the order in which they were observed in the field during this epizootic.