The north-east of Scotland (Grampian Region) has undergone three incidence and prevalence surveys, including the present one, since 1970. Results from these indicate a true increase in the prevalence of the disease in the region. The incidence of the disease has remained continuously high and shows a slightly upward trend. Literature on the subject of repeated surveys in different regions of the world has been reviewed in detail. The need for a prevalence study from the south of the British Isles has been emphasised in order to enable one to judge if the increase in Scotland is in keeping with the pattern in the whole of the British Isles. The familial incidence of the disease was noted to be virtually unchanged between the three surveys. Certain other aspects of aetiological significance have been analysed, viz, clustering of patients at birth or at onset of the disease; ages of occurrence of childhood viral infections such as measles, mumps, chickenpox and rubella; and the role of canine distemper infection.