Stigma has grave consequences for persons living with HIV/AIDS. Stigma hampers prevention of HIV transmission to sexual partners and to unborn babies, diagnosis, and early treatment, and negatively affects mental and physical health, quality of life, and life satisfaction. Internalized stigma of HIV/AIDS may have even more severe consequences than perceived or enacted stigma. The purpose of this study was to develop an instrument to measure internalized stigma in those with HIV/AIDS. Data were drawn from the Rural Women's Health Project. Research assistants administered structured interviews at baseline, 3 months, and 6 months. Instruments used in these analyses included a demographic data form, the Centers for Epidemiological Studies Depression Scale (CES-D), the Perceived Stigma Scale (PSS), and the Internalized Stigma of AIDS Tool (ISAT). Exploratory factor analysis confirmed that the ten items of the ISAT measure a single factor that explains 88% of the variance in the construct. Internal consistency was demonstrated by a Cronbach's alpha of .91 (Time 1), .92 (Time 2), and .92 (Time 3). Convergent validity was supported with significant positive correlations with the CES-D (rho = 0.33, p < 0.0001) and the PSS (rho = 0.56, < 0.0001). The Internalized Stigma of AIDS Tool appears to be a reliable and valid instrument to measure internalization of the stigma of HIV/AIDS. It may be of value in research and clinical assessment. © 2011 Informa Healthcare USA, Inc.