Born at Campana, Buenos Aires, Argentina on 27 August 1888, the son of Dr Michael John Petty, who practised there, he was educated at Prior Park School, Bath and Downing College, Cambridge where he took second class honours in the Natural Sciences Tripos, part I, 1909. He worked at King's College, London, and entered St Thomas's Hospital with the University scholarship in 1910, qualified in 1912, and took the Fellowship in 1914. He held house appointments at St Thomas's and served as resident medical officer at Lord Mayor Treloar's Orthopaedic Hospital at Alton. During the war he served as a Lieutenant in the RAMC and then returned to the Argentine, where he graduated MD at Buenos Aires University in 1918. He was appointed in 1919 surgeon to the model institute at the Rawdon Hospital and in 1921 assistant surgeon to the British Hospital in succession to J Wolfe Morris, becoming senior surgeon in 1931. After the Battle of the River Plate in December 1939, when a British naval squadron defeated a stronger German force which had taken refuge in the estuary, Petty organised a complete medical and surgical unit to aid the wounded in HMS *Exeter*, *Ajax* and *Achilles*, lying in extra-territorial waters. He went later to Boston to study R H Smithwick's technique in the treatment of hypertension by sympathectomy, and wrote a report on it in 1948. After his return to Buenos Aires he undertook research on cancer. He practised successively at 738 Juncal and 1636 Superi, and died at 1264 Basavilbaso, Buenos Aires on 20 January 1953 after a short illness aged 64. He had been created OBE in 1950 for his services in the war of 1939-45 and to the British community in the Argentine. He married in 1920 Lina M Pruden, who survived him with three sons and five daughters. Petty was a devout Roman Catholic; he founded the Consorcio de Médicos católicos and the Thomas More Society of Buenos Aires, and served as President of the latter. He was closely concerned in the welfare work of the British community. When the Eucharistic Congress met at Buenos Aires in 1934 he was the chief organiser of the medical bureau and first aid posts. Publications: Un cuerpo estranjo de la vejiga [a foreign body in the bladder], with A F Ortiz. *Rev Espec Asoc méd Argent* 1927, 2, 1077. Acute pancreatitis following ingestion of excessive amount of atophan. *Brit med J* 1928, 2, 442. Hipertensión esencial (obstrucción arteriolar) y la esplancnosimpatectomia dorsal de Smithwick. *Prensa med Argent* 1948, 35, 585.