A diagnosis of autoimmune lymphoproliferative syndrome caused by FAS deficiency during adulthood is unusual. We analyzed 17 cases of autoimmune lymphoproliferative syndrome caused by FAS deficiency diagnosed during adulthood in French reference centers for hereditary immunodeficiencies and for immune cytopenias. Twelve of the 17 patients had developed their first symptoms during childhood. The diagnosis of autoimmune lymphoproliferative syndrome had been delayed for a variety of reasons, including unusual clinical manifestations, late referral to a reference center, and the occurrence of somatic FAS mutations. The 5 other patients presented their first symptoms after the age of 16 years. In these patients, three germline heterozygous FAS mutations were predicted to be associated with haploinsufficiency and a somatic event on the second FAS allele was observed in 2 cases. Autoimmune lymphoproliferative syndrome may well be diagnosed in adulthood. The occurrence of additional genetic events may account for the delayed disease onset.