Professor Marc de Leval, a leading paediatric heart surgeon, established the heart transplant unit at Great Ormond Street Hospital, London, and was a pioneer of patient safety and risk management research. He was born on 16 April 1941 in the village of Charneux in Belgium, the eldest of seven children of Julien and Anne-Marie de Leval. His father and grandfather were both general practitioners. De Leval studied medicine at the University of Liege and qualified in 1966. He completed a residency in internal medicine and then trained at Liege for three years in general surgery. He decided to specialise in cardiac surgery and from 1970 spent two years as a fellow at the Pacific Medical Center in San Francisco under Frank Gerbode, a prominent cardiac surgeon. De Leval was subsequently appointed as a senior surgical registrar at Great Ormond Street Hospital in London, where he worked under David Waterston and Jaroslav Stark. He was awarded the Evarts A Graham Memorial travelling fellowship of the American Association for Thoracic Surgery and spent 1973 to 1974 at the Mayo Clinic in Rochester, Minnesota, where he worked with Robert Wallace, Dwight McGoon and Gordon Danielson. In 1974 he returned to Great Ormond Street as a consultant cardiothoracic surgeon. In 1988 he established the heart and heart and lung transplant programmes at the hospital. He also carried out the first paediatric heart transplant there. He chaired the hospital’s cardiorespiratory and critical care division from 2003 to 2005. In mid-career, after more than 50 successful arterial switch operations, he suddenly found that some of his young patients were dying following surgery. Showing exceptional humility, he decided to review his techniques, analyse his results and retrain, working with the statistician David Spiegelhalter. The 1994 paper they co-wrote concluded there was ‘an indication of suboptimal performance that appears to be neutralised by retraining’ (‘Analysis of a cluster of surgical failures. Application to a series of neonatal arterial switch operations’ *J Thorac Cardiovasc Surg*. 1994 Mar;107[3]:914-23). After retraining, 34 of his next 35 operations were successful. He argued constantly that surgeons needed to analyse their results. He researched innovative ways of reducing risk and increasing patient safety; he worked with Formula One to determine the management of pitstops, applying his findings to improve transfers from operating theatres, and studied data on plane ‘near misses’ collected by the Civil Aviation Authority. He was an adviser for the inquiry into child heart surgery at Bristol Royal Infirmary. He developed a series of innovative surgical procedures, including the ‘Great Ormond shunt’, used to treat a complex defect, ‘the blue baby with tetralogy of Fallot’. He published more than 300 papers and was editor of the influential textbook *Surgery for congenital heart defects* (London, Grune & Stratton, 1983), now in its third edition. He also wrote an autobiography *Humanity & humility: 40 years in children’s heart surgery*, published in 2020. After retiring from the NHS in 2006 at the age of 65, he continued in private practice but had to stop working in 2010 after being diagnosed with Parkinson’s disease. He was vice president of Chain of Hope, which provides heart treatment to children in 32 countries. In 2011 the American Association for Thoracic Surgery presented him with its scientific achievement award, its most prestigious honour. Outside medicine, he enjoyed fast cars and began each morning with a 5.30am swim. De Leval died from complications of Parkinson’s disease and cardiac and neurological issues on 26 June 2022. He was 81. He was survived by his wife Véronique ‘Vicky’ (née Laumont) and their two daughters Nathalie and Fabienne.