The "iatrotropic stimulus" is a typical example of Alvan Feinstein's terminology and his clinical way of thinking about medical research. Feinstein specified "interiatric referral" as an additional important factor affecting "the likelihood that patients will appear at the particular setting in which research is being conducted." He emphasized these phenomena as highly relevant for clinical judgment, clinical research, and clinical epidemiology. This has stimulated the realization of original clinical research in international primary medical care. It has helped to recognize the characteristics of the spectrum of health problem and morbidity patterns presented to primary care physicians and to acknowledge the position of generalistic primary care research between open population research and clinical research in referred (specialist) care. Accordingly, primary care-specific challenges in investigating (early) diagnosis, prognosis, treatment, and referral decisions have emerged. This research supports adequate decision-making by the primary care physician, which is important for effective health care. Although Feinstein was a clinical specialist and has always lived in a country without a very strong primary care tradition, he not only recognized the value of primary care research but also substantially contributed to its development.