Sir John Stuart Lilleyman (born 9 July 1945) is a British paediatric haematologist. His specialization is childhood leukemia and idiopathic thrombocytopenic purpura.
Lilleyman contributed to the testing of leukemia treatment methods and conducted research on "thiopurine metabolism" in children with leukemia.
He was president of the Royal College of Pathologists during the Alder Hey organs scandal. During that time, he was vice-chairman of the Academy of the Medical Royal Colleges.
In 2004, he became medical director of the National Patient Safety Agency (NPSA).
In 1991, the Royal College of Pathologists awarded him its first Distinguished Service Medal for establishing the United Kingdom's system of pathology laboratory accreditation. He was appointed president of the Royal Society of Medicine in 2004.