Children in foster care are considered a "vulnerable population" in clinical care and research, with good reason. These children face multiple medical, psychological, and social risks that obligate the child welfare and healthcare systems to protect them from further harms. An unintended consequence of the "vulnerable population" designation for children in foster care is that it may impose barriers on tracking and studying their health that creates gaps in knowledge that are key to their receipt of medical care and good outcomes. These gaps in knowledge have implications for justice, beneficence, and maleficence and serve to undermine "protection" of this population. Here we review the challenges of research regarding children in foster care, particularly medically complex children, and offer specific recommendations to include children in foster care in medical research.