In 1971–1972 he went as an Associate Research Scientist of the National Institutes of Health (United States) to the New York University Medical Center to work with Alan Johnson. In 1972–1973 he worked in the laboratory of Birger Blombäck at Karolinska Institutet in Stockholm (Sweden) as a NATO Research Fellow. After obtaining his PhD in Chemistry at the KU Leuven in 1974 and concluding his mandatory military service, Désiré Collen was appointed in 1976 as Lecturer (Docent) in the Faculty of Medicine at KU Leuven and as Adjunct Head of Clinic in the Section Bleeding and Vascular Diseases in the Department of Internal Medicine at the University Hospital Leuven.