Experience with twenty-five cases of acute encephalitis believed to be of viral origin has been analyzed. There is serological evidence that three of these cases were due to St. Louis virus. An unknown virus was isolated from the serum of one patient. In most of the cases, onset of illness was sudden. Fever, headache and mental change were the main symptoms. There was a mortality rate of 16 percent. It is significant that in about 35 percent of the cases, there was some residual defect after illness.