A specific disorder of sheep resulting in gestation periods extending up to eight months is reported for the first time. The condition, which is of low prevalence, occurs on hill farms in coastal regions of western Scotland and affected ewes usually die unless relieved of their fetuses by surgery. The fetuses may weigh up to 12 kg and have long hairy coats, skeletal deformities, precocious gonadal development and extensive liquefaction of central nervous system tissue. The aetiology has not been determined but some mechanisms which may be involved in the pathogenesis are discussed.