Influenza not being a notifiable disease in the United Kingdom, assessment of its prevalence is based on such indices as hospital admissions for pneumonia, new sickness-benefit claims, mortality statistics and reports from about 50 practices throughout the country submitted regularly to the Royal College of General Practitioners-all these supplemented by virological identifications performed by the Public Health Laboratory Service.From these sources it is shown that, compared with previous epidemic winters, the influenza prevalence in the winter of 1968-69 rose to only a moderate height in any one week, but extended over an unusually long period-from late December to early April. The total morbidity, as estimated from the excess new claims to sickness benefit, was similar to that of the winter of 1967-68, in which a sharp outbreak of influenza virus A2 infection occurred. The relatively leisurely progress of the 1968-69 epidemic was accompanied by no sudden or excessive demands either on general medical practitioners or on the hospital services. The mortality was substantially lower than in previous influenza winters.