Worcester, MA, experienced an outbreak of hepatitis during 1969-1970, an expected event which had occurred previously at eight-year intervals since reporting of the disease began in 1950. Other Massachusetts communities of similar character and the nation did not experience an epidemic during that same period. An extensive epidemiologic study of the disease illustrated that the epidemic followed the classical pattern in which individuals 5-14 years old were most affected irrespective of sex. During the interepidemic years from 1968-1972 in Worcester, and in all years (1968-1972) in both New Bedford and Springfield, MA, those primarily affected were young adults 15-30 years old, with male cases predominating. Sociodemographic statistical analyses also indicated the classical pattern of the less affluent, less educated, and sometimes the more crowded populations being at greater risk of contracting the disease. This outbreak of hepatitis was comparable to another in Greenland two years later in which immunologic methods differentiated between type A and type B viral infections. The data from both studies support the conclusion that the type B virus, often associated with parenteral drug use, is the predominate infectious agent during the interepidemic (endemic) periods. The type A virus is most likely responsible for the periodic epidemics.