Dennis Walder, professor of surgical science at the University of Newcastle, was a leading researcher in the field of decompression sickness ('the bends') and hyperbaric medicine. Surgery as a specialty has always been encompassed by wide boundaries, but Dennis' research must surely rank as a topic that stretched those boundaries to new limits. Dennis was born in Harrow on the Hill, Middlesex, the son of Horace Hampton James Walder, an architect, and Alice Wilhelmina Walder née Heide. He qualified from Bristol University in 1940. After military service, which ended in 1946, he secured surgical training posts in the North East of England, where he spent the remainder of his professional career until retirement. His interest in decompression sickness stemmed from his time as a medical officer in the wartime RAF, when he saw the problems faced by bomber pilots flying at great height without the benefits of pressurised cabins. He began studying why some pilots were more susceptible to sub-atmospheric compression sickness, and designed an apparatus to measure the surface tension of blood serum to help in these studies. In 1948 the Medical Research Council (MRC), faced with a high incidence of the bends in miners, divers and tunnel builders, invited Dennis and William Paton (later Sir William Paton, professor of pharmacology at the University of Oxford and professor of pharmacology at the Royal College of Surgeons) to monitor the health of workers working in compressed air as they constructed the pedestrian tunnel under the river Tyne. The results of their research set the standard for understanding and dealing with these workers' problems. In the sixties Dennis Walder masterminded the first code of practice for work in compressed air, which led, in 1996, to regulations for governing work in such conditions. Paton and Walder founded the Decompression Sickness Panel, of which Dennis was chairman for many years. From this grew the Decompression Sickness Registry, which kept copies of all the X-ray examinations of the major joints of compressed air workers. In the seventies Dennis won two major MRC grants to further the work of the registry, and led the transfer and computerisation of half a million records for over 4,000 workers. His studies demonstrated that the incidence of aseptic bone necrosis was directly related to the number of hours worked in compressed air. He set out to find why only some of the tunnel workers experienced health problems. In 1965 Dennis was awarded a Hunterian professorship for his research on problems relating to working in a hyperbaric atmosphere. He was awarded a MD and ChM, and became a member of the Faculty of Occupational Medicine. In 1966 he was promoted to professor of surgical science at Newcastle University. With the growth of diving activity in the North Sea in the search for oil, the clinical need for his expertise rose to high levels in the sixties. He would often be called out to oil rigs, sometimes in the night, to treat divers with the bends, and would transport the diver by boat or by helicopter back to the decompression chamber at Newcastle University, sometimes getting into the chamber himself to attend to the patient. He experimented on animals, mainly pot-bellied pigs, which have a similar physiology to humans, but did not exclude himself from such experiments. (He sometimes had to be admitted to the intensive care unit at the Royal Victoria Infirmary, Newcastle, when his experiments brought him close to death.) Even though he retired in the eighties, he continued to contribute through the Compressed Air Working Group. In 2002 he was the first medical practitioner to be awarded the James Clark memorial medal of the British Tunnelling Society for services to the tunnelling industry. Part of his citation read: 'The younger members of the Society will not appreciate how such a man could possibly contribute to our industry to the level that he is honored, but it is not for his skills as a surgeon that we owe him so much but for his continued quest to improve the lot of the compressed air worker.' Dennis married Winifred Osman Jones, a fellow graduate of Bristol University, in 1940. She died in 2005. They had two sons and a daughter. Dennis died on 4 September 2008, aged 91.