Medicine is a professional pursuit, and even in developing countries professionalism should lead at least some practitioners to care for their patients despite the absence of direct incentives to do so. Even if practitioners do not behave as professionals, what is the extent of latent professionalism, in which socialization in the profession conditions health workers to respond to a demand for professionalism even if they do not normally act as professionals? How many health care workers in developing countries act as professionals all the time and what will happen if health services turn toward remuneration schemes in which health workers are paid by the output or outcome? We examine the behavior of 80 practitioners from Arusha region of Tanzania for evidence of latent professionalism, professionalism and responsiveness to extrinsic incentives in the form of organizational demands for high quality care. We show that about 20% of these practitioners act like professionals and almost half of these practice in the public sector. Professional health care workers provide high quality care even when they work in an environment that does not reward this effort, a finding that has important implications for the use of performance–based incentives.