C F Regaud was among the first to study the biological effects of radium and X-rays, and their application to the treatment of cancer. Though not a surgeon he worked closely with surgeons, and always put the resources of his laboratory at their disposal. In recognition of his work in support of surgery he was elected a member and later an honorary member of the Académie de Chirurgie, and an Honorary Fellow of the College. Regaud was born and educated at Lyon, taking the MD in 1897 with a thesis on "The lymphatics of the testicle and the false endothelia of the seminiferous ducts". As a research histologist in the University of Lyon he founded the *Revue générale d'Histologie* (1904) and contributed to its first number a monograph on "The nerve-endings and sensitive organs of the locomotor apparatus". He was called to Paris in 1913 to become the first director of the radio-physiological laboratory founded by Marie Curie in the Radium Institute, with a professorship at the Pasteur Institute, but during the war of 1914-18 he served in the Military Health Service and became director of the School of War Surgery at Bouleuse. He recorded his experiences in a large volume of *Lectures on War Surgery* published in 1918. He was elected to the Académie de Médecine in the Institut de France in 1924, and visited Canada and the United States that year. From 1925 to 1928 he collaborated with A M B Lacassagne in a series of influential researches on the radiotherapy of cancer and the effect of radiation on normal tissues. Their results were published in three technical monographs and a textbook. He later published memoirs of Emile Roux (1933) and of Madame Curie (1934). He had many distinguished pupils. Retiring to Lyon before the second outbreak of war, he died there late in 1940, aged 70, survived by his son. Having been born before the Franco-Prussian war, he had seen his country overrun three times by the Germans. Though somewhat reserved he was always friendly and helpful.