Although its origin is uncertain, the 1968 influenza epidemic in Hong Kong may have spread from the mainland of China. It began in Hong Kong on 13 July and reached its maximum intensity in 2 weeks, lasting some 6 weeks in all. About 15% of the population was affected, but the mortality rate was low and the clinical symptoms were mild. The causative strain was isolated on 17 July and, because of its antigenic deviation from 1967 A2 strains, was sent to the World Influenza Centre in London and the International Influenza Center for the Americas in Atlanta, Ga., It was then proved to be a distinct antigenic variant of A2 virus, and the World Health Organization warned of its possible world-wide spread on 16 August.The rapid spread of influenza virus variants within and from Hong Kong is facilitated by the overcrowding of the population and by the constant communication both with the Chinese mainland and with the rest of the world. The population density is such that an epidemic can occur even in the hot subtropical summer.