This report, based largely on 1258 laboratory proven cases of infectious mononucleosis (IM) detected in the North-Eastern Regional Board area of Scotland during the years 1960–9 inclusive, describes and discusses some of the epidemio-logical and diagnostic aspects of the disease.

During the period of study, the annual rate of incidence increased fourfold from 11·1 to 44·3 cases per 100,000 population. Evidence is presented to indicate that this does not represent a true increase in prevalence but reflects improvements in disease detection. The annual incidence rates as reported by the C.D.S. for 1967–9 inclusive are regarded as a considerable underestimation of the ‘true’ incidence of the disease. Over a 3-year period an annual incidence equivalent to 100 per 100,000 population was found in each of two selected group practices. This value was constant and is thought to approximate the ‘ true’ incidence of symptomatic, seropositive IM in the general community.

Of the 1258 seropositive cases, 48·7 % were males and 51·3 % females. The peak age incidence was 15–19 years for both sexes. Twenty-one per cent of the cases were in children under 15 years and only 8·1 % in adults older than 25 years. No significant variation was found in the seasonal or urban/rural incidence. Marked differences were found in the leucocyte patterns of seropositive and ‘seronegative’ cases.