A Hidden Picasso is an unusual exhibition held to mark the 20th Congress of the International Institute for the Conservation of Historic and Artistic Works. A highlight of the Congress will be the presentation of the results of a research project into two works done by Picasso in 1900. During research on the painting Rue de Montmartre in the San Francisco Museum of Modern Art, x-rays revealed underneath a preliminary sketch of a Parisian nightclub bearing a surprising resemblance to Le Moulin de la Galette now in the Solomon R. Guggenheim Museum in New York.