Public health systems are dominant in much of the world, but often face fiscal constraints that lead to rationing of care. As a result, private sector healthcare providers could in theory beneficially supplement public systems, but evaluating the benefits of private alternatives has been challenging. We evaluate a private supplement to the free public health system for one of the world’s deadliest health problems, diabetes. We estimate enormous impacts of the private supplement, increasing the share of those treated who are under control by 69%.