Among the observations of patients suffering from abnormal movements, Jean-Gaspard Itard (1775–1838) published the case of Madame D. in 1825. It was republished in 1885 as the first clinical case characteristic of the disease described by Georges Gilles de la Tourette in the seminal article leading to his eponym, still in use today. However, the actual identity of Madame D., known throughout the 19th century as the Marquise de Dampierre, has remained a mystery, until now. The 17 July 1884 edition of the literary periodical Gil Blas provided an important lead by detailing the behavioural disturbances in society of the “Countess Picot de Dampierre”. Information from diarists at that time make it possible to confirm that this patient, known for her involuntary verbal outbursts, typical of coprolalia, in salons frequented by the 19th-century Parisian aristocracy was in fact Ernestine Émilie Prondre de Guermantes, her maiden name. She was born on 22 August 1800, and her married name was Countess Picot de Dampierre. She died on 08 July 1884. This article examines the life of this woman, her disease, her identification and the connection with the Duchesse de Guermantes, heroine of La Recherche du temps perdu written by Marcel Proust.