This conclusion concerns us because the study relied on rates of HIV diagnosis captured in Medicaid claims data. The article provides no information about the number of enrollees with severe mental illness, with or without a substance use disorder, who were tested for HIV in the first place. The authors underemphasize the potential detection bias this missing denominator creates in determining the incidence of HIV infection among all patients with serious mental illness, and especially among those without a substance use disorder. A review of the HIV testing literature found that among people with serious mental illness, those with drug problems or a history of injection drug use were more likely to be tested for HIV (2), and there is no reason to assume otherwise in the study in question. Furthermore, because the authors were unable to obtain data on risk behaviors, their adjustment for other factors associated with HIV infection among patients with serious mental illness was incomplete, regardless of substance use disorder. Therefore, they have constructed a confounding dichotomy and provided analyses that may lead providers and policy makers to incorrectly conclude incorrectly that patients with serious mental illness are not at increased risk of contracting HIV/AIDS.