357 cases of acute hepatitis occurring in 1974 in two distinct areas of Eastern Switzerland have been analyzed. The incidence of hepatitis was 54,8 per 100 000 per year in an urban region (Winterthur, pop approx. 95 000) and 33,7 per 100 000 per year in a rural area (St. Galler-Oberland, pop 47 000). These figures are three to four times higher than those reported by the public health authorities. More than 50% of the hepatitis patients were 16-30 years old and only 3% were children below 15 years of age. Males were slightly in excess. HBSAg (Hepatis Bsurface-Antigen) was detectable by radioimmunoassay in 49% and anti-HBS alone in 8% of all patients. According to a careful anamnestic survey the mode of transmission was thought to be parenteral in 137 (39%) of the patients (68% of these being HBSAg positive), enteral in 131 (38%) (38% of these being HBSAg positive), while no anamnestic clue was found in the remaining patients (23% with 39% HBSAg positives). In 12% of the hepatitis cases, contact with other diseased individuals at the relevant time was recorded; however, no clusters of more than 2 patients occurred within the study area. The results are discussed in regard to other studies concerning hepatitis epidemiology in Switzerland. In an additional survey the medical staff of several laboratories have been repeatedly tested for HBSAg and anti-HBS at intervals of one to three months. Transitory antigenemia (up to 55% positive in one laboratory with only one hepatitis case), and transitory high incidences of anti HBS (up to 40% in one laboratory) suggest epidemic spread of hepatitis B infection. From all available data it is estimated that the ratio of hepatitis B infection to overt disease varies between 1:1 and 23:1 depending on the epidemiological situation.