Fas and Fas ligand (FasL), members of the tumor necrosis factor (TNF) and TNF-receptor (TNFR) families of molecules, are involved in apoptosis. They are expressed in membrane-associated as well as soluble forms (sFas, and sFasL). Apoptotic defects underlie some models of autoimmune diseases, and they have been proposed in the pathogenesis of systemic lupus erythematosus (SLE) a prototypic autoimmune disorder. We measured the serum levels of sFas and sFasL in a series of well characterized SLE patients and devised an index of the two forms which resulted to be associated with age, indicating that apoptosis resistance is modulated during aging, thus explaining the conflicting observations made in previous studies.