Fas is an apoptosis-signaling receptor important for homeostasis of the immune system. In this study, Fas-mediated apoptosis and Fas mutations were analyzed in three Japanese children from two families with a lymphoproliferative disorder characterized by lymphadenopathy, hepatosplenomegaly, pancytopenia, hypergammaglobulinemia and an increase in TCR alphabeta+ CD4- CD8- T cells. Apoptosis induced by anti-Fas mAb was defective in both activated T cells and B cells, and granulocytes from these patients. Truncated Fas receptor lacking the cytoplasmic death domain caused by a point mutation in the splice region of intron 7 were demonstrated in two siblings. A homozygous point mutation in the splice acceptor of intron 3 was found in the Fas gene of the third patient, which resulted in the skipping of exon 4 and complete loss of Fas expression. Corresponding to these mutations, soluble Fas concentrations were decreased and reciprocally soluble Fas ligands were increased in patients' sera. Interestingly, co-stimulation by immobilized anti-Fas mAb in T cells from the two siblings was comparable to that seen in normal T cells. These results suggest that Fas-mediated apoptosis plays a pivotal role in immunological homeostasis in vivo, especially regarding clonal deletion of immune cells in humans.