His started working at the International Agency for Research on Cancer (IARC), placed in Lyon, in France in 1988. It was there that he met Elio Riboli and came in touch with the European Investigation into Cancer and Nutrition (EPIC) study in the early 1990s. From its earliest phase, Rudolf Kaaks has contributed to the planning and the development of this innovative project, that has revolutionised the epidemiology and our knowledge of the relationship between cancer, lifestyle, and diet. Later he led a research group on hormones and cancer at IARC, before moving to Germany, where he became the Principal Investigator of the EPIC study centre in Heidelberg.