Early antiretroviral therapy (ART) initiation reduces the risk of disease progression and HIV transmission, but data on time from HIV care entry to ART initiation are lacking. Using data from the Medical Monitoring Project (MMP), a population-based probability sample of HIV-infected adults receiving medical care in the United States, we assessed time from care entry to ART initiation among persons diagnosed May 2004-April 2009 and used multivariable Cox proportional-hazards models to identify factors associated with time to ART initiation. Among 1094 MMP participants, 83.9% reported initiating ART, with median time to ART initiation of 10 months. In multivariable models, blacks compared to whites [hazard ratio (HR) 0.82; 95% confidence interval (CI) 0.70-0.98], persons without continuous health insurance (HR 0.82; CI 0.70-0.97), heterosexual women and men who have sex with men compared to heterosexual men (HR 0.66; CI 0.51-0.85 and HR 0.71; CI 0.60-0.84, respectively), and persons without AIDS at care entry (HR 0.37; CI 0.31-0.43) had significantly longer times to ART initiation. Overall, time to ART initiation was suboptimal by current standards and significant disparities were noted among certain subgroups. Efforts to encourage prompt ART initiation should address delays among those without health insurance and among certain sociodemographic subgroups.