The aetiology, pathogenesis and epizootiology of African horse sickness (AHS) are reviewed with special reference to recent outbreaks in the Iberian peninsula. AHS is a highly fatal insect-borne viral disease of Equidae. It is caused by an Orbivirus (family Reoviridae) and nine serotypes are recognised. Outbreaks occurred in central Spain in 1987 and in southern regions of the Iberian peninsula in 1988, 1989 and 1990. All were associated with serotype 4 of the virus, whereas other occurrences of AHS outside Africa have all been caused by serotype 9. The clinical picture in the outbreaks was mainly of the acute (pulmonary) form except in 1988 when the subacute (cardiac) form of disease predominated. Several hundred horses died or were destroyed as a result of the outbreaks. Further spread was contained by a combination of slaughter of sick animals, movement controls, and vaccination which was extended over an increasingly wide area in successive years. The 1987 outbreak is believed to be associated with infected zebras imported from Africa. Possible explanations for the recurrence of disease in Spain in successive years are considered to include (a) the climatic conditions in Southern Spain, which could permit continuous vector activity, (b) the relative clinical resistance of mules and donkeys, which may permit subclinical circulation of the virus, (c) incomplete population immunity among horses due to possible gaps in the vaccination strategy.