In late 1969 an outbreak of hepatitis occurred in the mental retardation unit of a large mental institution. Six percent of the residents (45 of 780) were clinically diagnosed as having hepatitis during this period. Hepatitis-associated antigen (Australia antigen) studies made one year later on this same population and examined for relationship to diagnostic category, age at admission, length of residence, sex, race, ward, serum enzyme levels, immunoglobulin levels, and prior history of hepatitis showed positive correlation for sex (males, 49 of 51 positives), diagnosis (Down's syndrome patients were carriers at twice the rate of the rest of the population), and serum enzyme levels (SGOT significantly elevated in carriers as compared to other patients). No correlation was found with prior history of hepatitis or any other variable considered.