Since October 1949 Werner Forssmann was a regular guest of Prof. Dr. Hugo Wilhelm Knipping in the Medical University Clinic of Cologne. Established himself as urologist in Bad Kreuznach, Werner Forssmann had read about the American further development of heart catherization, which was invented by himself. Prof. Wilhelm Bolt, who was one of the medical station doctors of the Cologne Clinic, had already learned the technique of heart catherization in 1947. Thus, it was routinely performed in patients at the Cologne University Hospital. A close collaboration between Werner Forssmann and our research group (Hugo Wilhelm Knipping, Wilhelm Bolt, Helmut Valentin, Helmut Venrath, Hans Rink, Wildor Hollmann) was established. After the notification that Werner Forssmann had been awarded the Nobel Prize for Medicine in 1956, Hugo Wilhelm Knipping instructed me to help Werner Forssmann with the preparation of his lecture. Details of events in the year 1956 are illustrated. One of the important developments in which Werner Forssmann participated with the Medical University Clinic of Cologne was the selective pulmonary angiography.