Huw Llewelyn graduated from the University of Wales in 1970 with a degree in medicine. In 1975, he became a senior consultant and lecturer in medicine and endocrinology at St Bartholomew's Hospital in London. He was appointed a specialist at King's College Hospital in London in 1979. His doctoral thesis was supported by Nobel Prize winner Sir James Black and describes new theorems in probability theory that enable the systematic design and evaluation of tests for differential diagnostic assessment, as well as the identification of patients who are likely to respond or not respond to specific treatments. His clinical and teaching experience is based on work in university teaching hospitals in urban areas, in rural general hospitals, and in general practice. He is currently adapting the Oxford Handbook of Clinical Diagnosis for use by aspiring doctors at Kettering General Hospital.