Last year 2006, we commemorated two important events in the history of Neuroscience. One hundred years ago, on November 3, Alois Alzheimer (1864-1915) presented the first case of a patient with symptoms of a disease that later would be called Alzheimer's disease. One month later, on December 10, Santiago Ramón y Cajal (1852-1934) and Camilo Golgi (1843-1926) received the Nobel Prize "in recognition of their work on the structure of the Nervous System". These facts seem not to be related, but working in the Museum Cajal we found 37 histological preparations of material from patients suffering from Alzheimer's disease, revealing that Cajal also studied this disease. This paper deals with Cajal's contribution to the study of Alzheimer's disease and it is fully illustrated by original pictures of Cajal's slides preserved in the Cajal Museum, Madrid.