Cases of acute encephalitis reported to the CDC are divided into five groups: arboviral (8% of the 1965 to 1974 total), enteroviral (2%), post-infection (25%), encephalitis due to other known agents (3%), and encephalitis of indeterminate etiology (62%). With increased use of live virus vaccines against measles, mumps, and rubella, postinfection encephalitis has decreased, and SLE had become one of the most common preventable encephalitides in the United States. In 1975 SLE virus caused at least 1,791 cases of encephalitis, 42% of the reported total. In addition, the age distribution of persons with encephalitis of indeterminate etiology suggests that SLE virus may be an important contributor to that caegory during the summer months. The warm-weather transmission cycle of the SLE virus is well established. The reservoir is birds. The principal vector is the peridomestic C. pipiens mosquito in the Midwest and South and the rural C. tarsalis in the West. Man is an incidental and dead-end host. The winter reservoir is unknown. Human illness occurs in the summer. Asymptomatic human infections are about 200 times more common than symptomatic infections. Clinical attack rates and severity of illness increase with age. Case-fatality ratios of 35 to 38% have been reported for persons 60 years of age and older. For unknown reasons, SLE virus causes periodic major epidemics. The epidemics are more noticeable and better studied in major cities, but they probably affect rural areas as well. SLE is more common in areas of the country with warm climates. Epidemics in the North, when they occur, begin later but are of the same duration as epidemics in the South. Presumably, large epidemics of SLE can be prevented by mosquito control programs. Cumbersome and possibly insensitive diagnostic techniques impair our evaluation and understanding of SLE and other encephalitides. Insufficient information about the factors causing or preceding SLE epidemics impedes successful preventive measures. The use of emergency mosquito control programs after an epidemic has started has not been shown to reduce the number of human cases.