This study. initiated in June 1987. describes the epidemiology of bovine spongiform encephalopathy(BSE), a recently described novel neurological disease of domestic catte first identified in Great Britain in November 1986. Records suggested that the earliest suspected cases occurred in April 1985. There was variability in the presenting signs and the disease course, but the majority of cases developed behavioural disorders, gait ataxia, presis and loss of body weight: pruritus was not a predominant sign. The form of the epidemic was typical of an extended common source in which all affected animals were index cases. The use of therapeutic or agricultural chemicals on affected farms presented no common factors. Specific genetic analyses eliminated BSE from being exclusively determined by simple mendelian inheritance. Neither was there any evidence that it was introduced into Great Britain by imported catte or semen. The study supports previous evidence of aetiological similarities between BSE and scrapie of sheep. The findings were consistent with of cattle to a scrapie-like agent, via cattle feedstuffs exposure containing ruminant-derived protein. It is suggested that exposure began in 1981/82 and that the majority of affected animals became infected in calfhood.