After returning to London from Aden, Kay worked for six years as a pathologist at
St Thomas' Hospital. He was appointed a consultant pathologist at the
Royal Marsden Hospital, a specialist cancer treatment hospital in London, in 1956. At the time, leukaemia was a fatal disease, usually causing death within weeks of diagnosis, and there was little consensus about treatment options. Kay organised and oversaw multiple clinical trials funded by the
Medical Research Council relating to leukaemia treatments and served as secretary to the MRC's Leukaemia Committee from its founding in 1968 to its disbanding to 1977. Other MRC trials that Kay coordinated at the Royal Marsden focused on
multiple myeloma and
polycythaemia rubra vera. He was editor of the
Journal of Clinical Pathology from 1972 to 1980. At the Royal Marsden, Kay helped to design and was the first administrator of an isolation ward for patients with
weakened immune systems that was built in 1963. The concept was so successful that a larger version of the ward was opened in 1973 and was equipped for intensive treatment of acute leukaemias. At the time of his retirement as a professor of haematology in 1984, leukaemia had become a largely curable disease. ==Later life==