Background: To assess rates of screening and testing of HIV and HCV among those with serious mental illness and co-occurring substance use disorders. Methods: One hundred fifty-three people with serious mental illness and co-occurring substance use disorders completed measures and were screened for HIV and HCV. Results: Six percent were HIV positive and 25% were HCV positive. Almost a quarter reported a history of injection drug use and 86% reported a history of unprotected sexual encounters. Compared to those without a diagnosis of Hepatitis C, those diagnosed with Hepatitis C were significantly more likely to have a sexually transmitted infection, (p=0.01), have a lifetime history of injection drug use, (p less than 0.001), and a lifetime history of sniffing drugs, (p=0.01). Conclusions: Given the high levels of infection of HIV and HCV and high levels of transmission risk factors efforts to improve screening and provide risk reduction counseling are warranted.