Sexually-transmitted amebiasis due to transmission of Entamoeba histolytica cyst by oral-anal sexual contact of homosexual men seems to be primarily responsible for the recent increment in prevalence of amebic infection in the western countries and Japan. Numerous epidemiological studies clarified some unusual aspects of this novel sexually-transmitted disease (STD). The most striking feature was the fact that virtually all of the E. histolytica isolates from homosexual men in the western countries were judged to be non-pathogenic by clinical findings, serologic investigations and zymodeme analyses. Accordingly, the homosexual men with amebic infection in these countries were essentially asymptomatic cyst carriers. However, in Japan, a number of homosexual men have been found from the cases with invasive amebiasis. This led us to investigate the pathogenicity of E. histolytica strains circulating among Japanese including homosexual men. First, we found a high correlation between positive syphilis serology with invasive amebiasis. Subsequent studies indicated that the seropositivity of Japanese homosexual men for amebic infection was much higher than that of heterosexual males and female prostitutes, and that the ameba isolates from homosexual men had zymodeme II, XIV and XIX, all of which stood for pathogenic E. histolytica. These findings strongly suggest that pathogenic strains of E. histolytica are spread among male homosexual communities in Japan. Because pathogenicity of this protozoon appears to be the most important factor for development of symptomatic, invasive amebiasis, these data seem to be reasonable enough to explain why in Japan we have had a number of cases with invasive sexually-transmitted amebiasis. Zymodeme XIV was also frequently detectable from the cases without the history of homosexuality, which leads us to envision that the unique epidemiological aspects of amebic infection in Japan may be relevant with Indian Subcontinent, virtually the only region where this zymodeme has been frequently found.