Suppose that someone has a serious illness. The illness will likely lead to significant disabilities, and may even cause death. Existing treatments are unsatisfactory. The patient learns about a clinical trial, in which some allegedly promising new treatment for that illness is being tested.Such seriously ill patients for whom existing treatments are unsatisfactory have sometimes been categorized as medically vulnerable in the literature. Should these patients indeed be considered vulnerable subjects and be provided with special protections? And if the answer (...) is yes, then what are those special protections? This article explores the possible answers to these questions.The federal regulations for the protection of research subjects have several provisions addressing the concept of vulnerability. The three subparts Subparts B, C, and D deal with specific categories of vulnerable subjects, but do relatively little to address broader issues relating to vulnerability that might apply to a category such as the medically vulnerable. The Common Rule is the only section containing general discussions of vulnerability. (shrink)