Niels A. Lassen – elite clinical physiologist and neuroscientist. Niels A. Lassen, a Danish physician, was a very successful neuroresearcher in the last half of the twentieth century. In addition to determination of cerebral blood flow, regional cerebral function, brain mapping and functional brain imaging, he contributed substantially to circulatory-, pulmonary- and renal physiology, pathophysiology and clinical physiology, and to kinetic theory, nuclear medicine and molecular imaging techniques. His achievements were obtained by a combination of dexterous experimental skills, hard-working, biological intuition and mathematical intelligence. Modern bibliometric analysis has confirmed Niels A. Lassen’s unique contribution to several biomedical research fields.