Human immunodeficiency virus (HIV)-infected individuals are at high risk of malignancies. However, it is not currently the standard of care to routinely test cancer patients for HIV. In 2006, the Centers for Disease Control and Prevention recommended HIV testing in all health care settings, calling for standard nontargeted "opt-out" HIV screening. For a variety of reasons, routine opt-out HIV testing is still not widely used in the United States. Although many barriers to routine opt-out HIV testing have been addressed, such opt-out HIV testing continues to be conducted primarily in venues that target specific patient populations such as pregnant women. Although opt-out testing has been piloted in emergency departments, less emphasis has been placed on opt-out HIV testing in other clinical settings. In this article, the background, rationale, and evidence for supporting opt-out HIV testing as routine care for cancer patients are presented. In addition, evidence is discussed for the potential of opt-out HIV testing to improve clinical outcomes by facilitating appropriate HIV management during cancer treatment for individuals who are found to be HIV positive.