Prospective virologic surveillance has defined two influenza epidemics representing the fifth and sixth outbreaks attributed to H3N2 viruses since the prototype, A/Hong Kong/68 ((H3N2), emerged in 1968. The 1975 epidemic was caused by influenza A/Port Chalmers and yielded an estimated attack rate of 9 per cent; the second, attributed to influenza A/Victoria, produced an explosive outbreak, with an estimated attack rate of 18 per cent in 1976. The highest morbidity occurred in preschool children, with an estimated attack rate of over 30 per cent. During the early stages of both epidemics there was a predominance of cases among school-aged children, and school absenteeism peaked earlier than other nonvirologic indexes. These observations support the concept of rapid dissemination of influenza among schoolchildren and suggest that control of epidemic influenza might be facilitated by prophylaxis for that age group and other accessible, healthy populations.