6246 infectious hepatitis cases treated at 25 hospitals located in Okayama Prefecture during the last six years, 1952 to 1957, were studied mainly from the epidemiological point of view, and the following results were obtained. 1) Infectious hepatitis has been prevailed all over the prefecture since the initial epidemic in 1951. As the mode of epidemic became protracted, chronic and abortive forms were prone to increase in number. The ratio of total patients to infectious hepatitis cases was 1.27 per cent. 2) Extensive infections among villagers were noted in 73 hamlets, and it was presumed that the epidemic has been prevailed over the whole prefecture taking a hamlet, village or a town as unit. Infections among family were noted in 62 families, however, fnrther detailed study in the epidemic areas might show the higher morbidity. It was not certain whether these infections were entirely family or simultaneous infections. 3) As to seasonal occurence, considerable number of cases had onset of the disease during the period of early summer to early fall. 4) As to age, it has ranged mostly from the second to third decade, rather many in children but not rare in the aged. As to sex, predominant in males in general, but considerable differences were seen among hospitals and times. No definite relationship was noted between the epidemc and sex. 5) Throughout the last six years there have been 38.7 per cent of chronic hepatitis cases. In 41.3 per cent of the whole patients presented jaundice in their clinical courses. The fact that the longer the period of the epidemic the more increase increase in anicteric cases was noted. 6) Two year's follow-up studies after the epidemic, showed 209 of relapsed cases in whom 36.4 per cent were icteric. 7) 110 cases has jaundice, which was thought to be infectious hepatitis, in the past. Some of them were presumably reinfections or superinfections but definite conclusion could not be made. 8) 20 cases of fulminant hepatitis, malignant type of infectious hepatitis, and 6 cases of protracted form were studied. 22 cases of them were fatal. Such severe hepatitis patients were seen mainly in males of the second and third decades. 9) 61 cases (0.98%) developed postnecrotic cirrhosis, and 10 cases of them were fatal. 10) Serum hepatitis was noted in 1.6 per cent of the whole patients seen in Medical Services at 18 hospitals in Okayama Prefecture during the last six years, 1952 to 1957. No definite correlation between the epidemic of infectious hepatitis and outbreak of serum hepatitis was noted.