In 1923, he provided an early correlation between lung cancer (Bronchialkarzinom) and tobacco smoking. Today his name is associated with Fahr's disease, which is a degenerative neurological disorder characterized by calcifications and cell loss within the basal ganglia.
Fahr is remembered for his work in nephrology and research of kidney disorders. With internist Franz Volhard (1872-1950) he published a comprehensive monograph on Bright's disease titled Die Brightsche Nierenkrankheit.