Historians of medicine--and even Louis Pasteur's biographers--have paid little attention to his close relationship with the Finnish artist Albert Edelfelt. A new look at Edelfelt's letters to his mother, written in Swedish and never quoted at length in English, reveals important aspects of Pasteur's working habits and personality. By understanding the active collaboration through which this very famous portrait was made, we also discover unnoticed things in the painting itself, gain a new appreciation of its original impact on the French public's image of science, and better understand its enduring influence on the portrayal of medicine in the art and the popular culture of many countries even to the present day.