After the war he studied for a Diploma in Public Health at the London School of Hygiene & Tropical Medicine and spent a year at the Henry Phipps Institute in Philadelphia on a Rockefeller Fellowship. Cochrane joined the Medical Research Council's Pneumoconiosis Unit at Llandough Hospital, a part of Welsh National School of Medicine, now Cardiff University School of Medicine in 1948. Here he began a series of studies on the health of the population of Rhondda Fach — studies which pioneered the use of randomised controlled trials (RCTs).