Patrick Mollison was a pioneer in blood transfusion, playing a major role in changing it from a risky procedure to one which is now extremely safe. The urgent need for blood during World War II provided a stimulus for the development of this important lifesaving measure. His first major contribution was to devise a mechanism whereby blood could be stored for more than just short periods. Mixing donated blood with acid–citrate–dextrose (ACD) became a standard procedure for almost 30 years and was used worldwide. He later took a special interest in haemolytic disease of the newborn (HDN), which was largely due to Rhesus incompatibility between mother and baby. He was also involved with work which eventually led to HDN becoming preventable with the use of anti-D treatment of mothers. He wrote the first standard textbook on blood transfusion; almost 70 years later it is in its eleventh edition and still bears his name in the title. He spent his working life in blood transfusion and the study of the scientific aspects of this subject, developing a university department at Hammersmith Hospital and publishing almost 200 scientific papers as well as the textbook. He was very much a clinical scientist rather than a front-line clinician, although he was physician to Her Majesty Queen Elizabeth and was present at the birth of all four of her children.