In 1979-1980, a distinct outbreak of hepatitis A occurred among homosexual men in Stockholm, Sweden, city and county area. The epidemic comprised 145 known cases. It began in December 1979 and progressed in waves during the following 10 months, with three distinct peaks separated by about six-week intervals. Actually, the incidence of hepatitis A in the Stockholm area showed a fivefold increase during 1980 as compared to the previous year. Clinical serologic, and social characteristics were studied more closely in 98 of the 145 homosexual men. Verification of hepatitis A was made by a solid-phase radioimmunoassay technique for detection of antibody to hepatitis A virus of the immunoglobulin M class. In addition, 64% of the men showed findings consistent with a prior hepatitis B (antibody to hepatitis B core antigen and/or antibody to hepatitis B surface antigen) and 34% were Treponema pallidum immobilization-positive from a prior or concomitant syphilis. Employment in risk professionals was common; thus, 19% worked in restaurants or otherwise handled food and 20% were engaged in medical care as compared to the 1% occupied in either branch of work among the general population in Sweden. Sexual habits with multiple partners and oral-anal sexual contacts were judged to be of major importance in the spread of this epidemic. Some spread of hepatitis A to the general population probably occurred due to the risk occupations of many homosexual men.