The Wiskott-Aldrich syndrome (WAS) is a primary immunodeficiency disease caused by mutations in the Wiskott-Aldrich Protein (WASP) gene, which typically leads to absent WASP protein expression in WAS leukocytes. However, some patients have been found with small populations of WASP-expressing cells caused by reverse or second-site mutations that allow protein expression. An international consortium was established to further investigate these phenomena. This paper summarizes data collected by this consortium that was presented at a workshop held during the XIIth Meeting of the European Society for Immunodeficiencies (ESID), October, 2006. WASP reversions were noted in approximately 11% of 272 patients tested. Many different cell lineages showed reversions. These data form the foundation for further investigation into this phenomenon, which has implications for therapy of this disease.