Observation of a patient with acquired hypogammaglobulinemia associated with a mononucleosis syndrome led to the identification of one of the largest families affected by the X-linked lymphoproliferative (XLP) syndrome in the world. It is the first such family identified in Switzerland and the largest in Europe. At least nine male subjects over two generations presented phenotypic expressions consistent with the XLP syndrome. Study of the pedigree extending over seven generations suggests that the mutation occurred in the proband's great-grandmother. In the next generation, a second mutation of the X chromosome in one branch of the family resulted in expression of hemophilia A in the children. This remarkably large family, comprising six living obligate female carriers, displays a wide spectrum of the XLP syndrome and offers valuable information for future genetic linkage studies and for genetic counseling.