We aimed to identify factors associated with a medical provider's resistance to prescribing HAART to medically-eligible HIV-infected illicit drug users. In four US cities, a mailed, self-administered survey queried 420 HIV care providers about patients' characteristics and barriers to care. Adjusted odds ratios (AOR) and 95% confidence intervals (CI) were estimated using logistic regression. Providers identified as resistant to prescribing HAART to medically-eligible HIV-infected illicit drug users were more likely to be non-physicians (AOR=1.89, 95% CI: 1.04-3.46), to work in populations with a high prevalence of both mental illness (AOR=2.42; 95% CI: 1.11-5.26) and injection drug use (AOR=1.82 95% CI: 1.02-3.25) and were deterred from prescribing HAART by patients' limited ability to keep appointments, (AOR=3.19; 95% CI: 1.39-7.37), alcoholism (AOR=1.92; 95% CI: 1.04-3.55) and homelessness (AOR=1.81; 95% CI: 1.07-3.06). Providers working in populations with a high injection drug use prevalence commonly reported higher prevalence of non-injection drug use, alcohol problems and mental illness, and higher antiretroviral therapy refusal rates within their patient populations. Our findings underscore the challenges to providers who treat HIV-infected drug users and suggest that their care and treatment would benefit from on-site drug treatment, mental health and social services.