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Objectives: To identify epidemiological characteristics of heterosexually acquired AIDS in Japan, with emphasis on potential influence on future trends., Methods: National AIDS Surveillance data in Japan were compared with those in the UK and US, where detailed information is available from well-established surveillance procedures. Data on AIDS cases diagnosed until the end of 1996, particularly those acquired heterosexually, were analyzed by year of diagnosis, gender and age group., Results: The number of heterosexually acquired AIDS cases in Japan has continued to increase, while those in the UK and US leveled out or decreased recently. The increase during a two-year period after reaching a certain number of cases per year was found to be 2.3-fold in Japanese, 2.4-fold in UK whites and 5-fold in US whites. The male to female ratio (M/F) for heterosexually acquired AIDS was 6.3 among Japanese, while the ratio was 1.1 and 0.5 in the UK and US, respectively. The age distribution at AIDS diagnosis demonstrated a peak from 35 to 54 years of age among Japanese males, as compared to 30 to 34 among males in the UK and the US. No significant difference was apparent in the age distribution among females in the three countries., Conclusions: The relatively small number of AIDS cases in Japan is attributable to the late introduction of HIV and the limited chance of heterosexual transmission from homosexual/bisexual men and injecting drug users. In addition, transmission has probably most often occurred between middle-aged Japanese males and non-Japanese females. As there is a growing risk of HIV infection among Japanese females and young Japanese males, new prevention strategies targeting these groups are urgently required.