Striking advances in HIV prevention have set the stage for renewed debate on setting priorities in the fight against HIV/AIDS. Two new prevention strategies-preexposure prophylaxis and treatment as prevention-use antiretroviral drugs for prevention of HIV/AIDS in addition to treating patients. The potential for success of these new prevention strategies sets up an ethical dilemma: where resources are limited and supplies of lifesaving antiretroviral medications are insufficient to treat those currently living with HIV, how should these resources be divided between treatment and prevention? This article explores several ethical principles used in formulating public health policy. Assuming that limited resources are available for spending on drugs, we conclude that it would be unethical to watch patients with treatable AIDS worsen and die, even with supportive care, so that medications for treatment can be diverted for prevention.