DURING the summer of 1945 an outbreak of encephalitis occurred on Okinawa Shima. At the time, the island was the most important advanced base in the Pacific theater. Large forces of American troops were already assembled for the projected invasion of the home islands of Japan, scheduled for the autumn. It was reasonable to assume that these troops were susceptible to an oriental neurotropic virus disease, and the danger of an epidemic was a matter of great concern to the medical departments of the Army and Navy. When the first civilian patients with encephalitis were reported to the island surgeon, a program for their isolation and study was instituted at once. Within a very short time several groups of investigators were engaged in observing various aspects of the disease and active measures were being taken for its control. Hospital facilities were set up by the Military Government Research Center for