Autoimmune lymphoproliferative syndrome (ALPS) is characterized by chronic, histologically benign splenomegaly and generalized lymphadenopathy, hypergammaglobulinemia, and autoantibody formation. ALPS has been attributed to defective programmed cell death of lymphocytes, most often arising as a result of mutations in the gene encoding the lymphocyte apoptosis receptor Fas/APO-l/CD95. We identified a novel mutation in the intracellular apoptosis signaling domain of Fas in 11 members of a family, individual members of which have been monitored for up to 25 years, with 1 or more features of ALPS. This study of a large number of family members carrying the same Fas defect demonstrates that ALPS is inherited in an autosomal dominant fashion but with a high degree of variability in clinical expression. Although 1 affected individual died of postsplenectomy sepsis and 1 has been treated for lymphoma, the Fas mutation in this family has been compatible with a healthy adulthood, as clinical features of ALPS have receded with increasing age.