Research involving the vulnerable sick raises difficult challenges for investigators and Institutional Review Boards. Exactly who among the ill counts as vulnerable is a matter of judgement, and involves consideration of susceptibility to harm and capacity to provide free and informed consent. A balanced approach is required when protections are considered, and the benefits as well as the risks of research participation must be carefully weighed. A variety of protections for the vulnerable sick in research are available, including enrolling subjects in a study only with a strong justification, ensuring that consent is free and comprehending, and setting limits on the risk to which they may be asked to endure. Discussion of three contemporary controversies, placebo-controlled trials in the psychiatric setting, phase I clinical trials and oncology patients, and emergency room research involving patients incapable of giving consent, highlights the little recognized prominence of risk-benefit issues in the clinical studies in the vulnerable sick.