Optimistic thought regarding one's own future is said to be an adaptive and mentally healthy personality. Except for dispositional optimism, some situation-specific optimistic expectations have been observed as a psychological adjustment when people are faced with threatening events, such as disease. In this study, we examined the development of an optimistic prospect for AIDS with people living with HIV under antiretroviral therapy in Japan. Participants rated relative probabilities of how likely they, compared to others who were in similar circumstances including HIV diagnosis, might have one of four kinds of disease in the future, cancer, cerebral apoplexy, myocardial infarction, and AIDS. The participants demonstrated a significantly lower prospect for progression to AIDS than other people living with HIV receiving treatment at the same hospital, while their prospects for the other three diseases were rated at the same level as those of other people living with HIV. From exploratory analysis, this optimistic outlook regarding the progression to an AIDS diagnosis was associated with individuals' causal attributions; those who did not believe forces beyond human understanding could bring about the desired result exhibited a more optimistic expectation for AIDS. These findings suggest that people living with HIV under antiretroviral therapy in Japan have a situation-specific optimistic expectation about AIDS, and this optimism is influenced by their attribution style regarding their health.