This study is to identify the risk behaviors of the Japanese that may lead to HIV infection and the behaviors that prevent such infection, as well as their background factors. Two behavioral surveys were conducted for the present study. (1) For international comparison on knowledge, attitudes, beliefs, and practices (KABP) related to HIV/AIDS, we conducted a survey on a sample of 10,000 adults, randomly selected from a nationwide population in Japan; and (2) for sexual partner relation, we conducted a survey on a sample of 10,000 adults randomly selected from a population in five major cities of Japan. Our main findings include: (1) Most of the Japanese adults did not regard AIDS as a major threat in the area where they lived; (2) People in their twenties are too casual about ''having sex without using a condom with someone they've met for the first time and know little about''; (3) Thirteen percent (19% male, 8% female) of those with a steady sex partner including a spouse, on average, had sex with 2.4 non-steady partners in the previous year; and (4) Only 25 percent used condoms always when they engaged in casual sex during the previous four weeks. These risk behaviors of the Japanese adults might lead to an explosive rise in the number of HIV-infected in the near future unless steps are taken immediately to prevent it.