Private commercial providers represent a growing source of health care in Lagos State, Nigeria, serving people from all socioeconomic classes. While 65 percent of commercial providers in Nigeria offer HIV testing and counseling, just 10 percent provide antiretroviral therapy (ART). Increasing the role of the commercial sector in ART provision could help reduce unmet need, easing the burden on the public sector. To stimulate a more informed discussion about the potential role of the private sector, we estimated the magnitude of the potential private sector contribution in Nigeria and identified barriers to expansion of the private sector's role in ART delivery. We developed a series of equations to estimate the number of additional people living with HIV who could plausibly receive ART if commercial sector involvement were strengthened. Facility surveys and stakeholder interviews were conducted to measure interest in providing ART, the capacity to provide it, and perceived barriers to its provision. We found that commercial providers have substantial slack capacity that could be used to provide ART. The primary barriers to scaling up ART provision were inadequate provider expertise, laboratory capacity, and financing the cost of treatment. If these barriers were removed, the commercial sector could fill 97 percent of the estimated unmet need for ART in Lagos State. Strengthening the capacity of private providers to meet the needs of people living with HIV offers a promising path to achieving universal ART coverage in Nigeria.