Born at Glenridding Westmoreland on 20 October 1880 he was the eldest son of Francis Charles MacNalty MD, MCh, sometime senior assistant physician to the Metropolitan Hospital, London, and Hester Emma Frances, nee Gardner, who was the grand-daughter of Sir John Piozzi Salisbury. MacNalty's boyhood was spent in the Lake District and Winchester where his father worked after leaving London. He was educated at Hartley College, Southampton, and later became a member of Corpus Christi College, Oxford. He completed his medical education at University College Hospital, London. After holding resident posts at his hospital he became resident medical officer to the Brompton Hospital, then medical registrar to the London Hospital (1911-1913). While at University College Hospital he worked with Sir Victor Horsley on the cerebellum and a paper on this research appeared in Brain in 1909. He also investigated heart block with Thomas Lewis; their joint paper in the *Journal of physiology* (1908) recorded for the first time the use of the electrocardiograph for the diagnosis of heart disease. In 1913 MacNalty's career was diverted to preventive medicine when Sir John Burns, MP offered him the medical inspectorship at the Local Government Board. In this appointment he was employed in measures to combat tuberculosis. During the first world war he was seconded to the War Office and worked with R S Reece and Sir Shirley Murphy on the outbreaks of cerebro-spinal fever amongst troops and civilians, and they also confirmed Wickman's findings on contact infection in poliomyelitis. From 1919-32 MacNalty was deputy senior medical officer of the Ministry of Health and secretary of the Tuberculosis Committee of the Medical Research Council during which time he published several papers on tuberculosis/poliomyelitis and encephalitis lethargica. From 1932-34 he was senior medical officer for tuberculosis and deputy chief medical officer to the Ministry of Health under Sir George Newman, becoming chief medical officer in 1935. From 1935 until the outbreak of the second world war he was one of a medical advisory committee to the Ministry of Health which among its members included Lord Dawson of Penn, Lord Moynihan, Lord Horder and representatives from the British Medical Association. At MacNalty's recommendation the Ministry set up a departmental committee to review amongst other things the conditions of service of the nursing profession and the medical aspects of the Midwives Act of 1936. He also persuaded the Ministry to make the purchase of anti-diphtheria vaccine free to the local authorities and thus practically eliminated diphtheria as a killing disease of children. In 1939 MacNalty was sent by the Minister of Health on a mission to Canada and the USA to inform the authorities there of our medical preparations in case of war and on his return he served as chairman of special committees to deal with various aspects of the Emergency Medical Service. In 1941 at the age of 60 he retired and was immediately appointed editor in chief of the official medical history of the second world war under the chairmanship of Mr R A, later Lord, Butler. He served on the Council of the Royal College of Physicians (1937-39) and also continued as Crown nominee on the General Medical Council until 1943. He was appointed honorary physician to the King from 1937-46. He became Milroy Lecturer to the College of Physicians (1925); Vicary Lecturer to the Royal College of Surgeons (1945) and Holme Lecturer at University College Hospital (1955). He also examined in public health for the Universities of Oxford, Birmingham and London. Amongst his other honours he was elected an Honorary Fellow of the Royal Society of Edinburgh (1949), a Freeman of the City of London and an Honorary Freeman of the Society of Apothecaries and of the Barbers Company. In 1963 he was elected a Fellow of the Society of Antiquaries. He also served as President of the Epidemiology Section of the Royal Society of Medicine being elected an Honorary Fellow in 1959. MacNalty was a small man with a modest demeanour with brilliant eyes and a charming voice. He possessed a profound and varied knowledge of science, history and literature and his vision and administrative ability achieved real advances for the nation's health. He married in 1913 Miss Dorothea de Wesslow and they had two daughters. His wife died in March 1968, and Sir Arthur died on 17 April 1969 at Bocketts, Downs Road, Epsom; one of his daughters survived him.