Résumé : Between 1945 and 1965 the Medical Faculty of Leiden University flourished. Internal Medicine, headed by Jacob Mulder since 1946 until his untimely death in 1965, was developed according to the model Mulder had observed during his visits to the U.S.A. in 1947. In the Netherlands, Internal Medicine had generally been practised without sub-specialities at the end of World War II. Mulder, however started to divide Internal Medicine into a series of sub-disciplines such as rheumatology, cardiology, haematology, immunology, infectious diseases, pulmonology, nephrology, gastro-enterology, endocrinology and metabolism. A large number of his co-workers became professors in their specialized field at Leiden Univesity, but he tried in vain to create a professorship in gastro-enterology for his friend A.J. Ch. Haex (1913-), who was controversial in the Medical Faculty. Nevertheless, Haex succeeded in getting the post of president (1966-1983) of the influential committee Gezondheidsraad which advises government on health matters. Mulder regarded the construction of a new Internal Medicine out-patient department according to the example of the Mayo Clinics in Rochester, U.S.A., as one of his major achievements. He also emphasized the importance of continuous further training. From 1957 onwards he organized courses for general practitioners, the well-known Boerhaave Courses. This initiative met a favourable response. These courses are still offered today.