Lionel Zelick Cosin, son of Benjamin Cosin, a tobacconist, and his wife Mary, née Magoon, was born in London on 8 November 1910. He was educated at Westminster City School and Guy's Hospital Medical School, qualifying in 1933. His brother, the radiologist Dr C F Cosin, was senior to him at Guy's. His house posts were at the Prince of Wales Hospital, Tottenham, and the Royal Northern Hospital, Holloway, under the supervision of Arthur Dickson Wright, Hamilton Bailey and W B Gabriel. He passed the FRCS in 1936. On the outbreak of war he was appointed medical superintendent in the Emergency Medical Service at Orsett Hospital, Essex. It was a move which was to change his life. As well as the daily coach convoys bringing the blitz casualties from London he had also to look after some 300 chronic in-patients. On inspection he found that many were suffering from treatable illnesses and more were in need of active rehabilitation. With enthusiasm and reorganization and with emphasis on an acute approach he achieved a high home return rate, so that only one in five patients was required to be in hospital for over six months. In 1948 he moved to Langhorn Hospital, East London, where he pioneered a day care unit for the mentally ill. In 1950 he was invited to become the Director of Geriatrics, a new specialty, at the United Oxford Hospitals and lecturer to the University and Magdalen College. He established his unit at Cowley Road Hospital and, developing the systems from Orsett, he reduced the average hospital stay from 286 to 51 days. He invented the concept of the 'floating bed' to allow respite short stay for patients. His name rests securely on the development of the geriatric day hospital and he planned the building of a new, purpose-built unit in 1957. This was renamed after him on his retirement in 1972. Lionel Cosin was a prolific and lucid writer, publishing over eighty articles, mainly on geriatric topics. He gained international renown, holding visiting professorships and lecturing to some ninety universities throughout the world. He was also a generous supporter of many Jewish charities. He died on 21 March 1994, survived by his wife Pamela, née Keenlyside, whom he had married in 1941 and their two children, Benjamin and Philippa.