Spontaneous apoptosis was observed in a proportion of peripheral blood mononuclear cells obtained from patients with head and neck cancer (HNC) but not from normal healthy donors (T. Saito et al., Clin. Cancer Res., 5: 1263-1273, 1999). To further investigate this phenomenon, peripheral blood mononuclear cells were obtained from patients with HNC or normal controls (NCs) and evaluated for expression of apoptosis markers (annexin V binding and caspase-3 activation), T-cell receptor-associated zeta chain, and the death receptor Fas (APO-1, CD95) in CD3(+) T cells by multicolor flow cytometry. Soluble Fas ligand (sFasL) in the sera of these individuals was quantitated by ELISA. In patients with HNC, 74 +/- 15% (mean +/- SD) of CD3(+) T cells were Fas(+) compared with 52 +/- 13% in NCs (P < 0.0001). Furthermore, 29 +/- 16% of the Fas(+) CD3(+) T cells bound annexin V in patients and only 14% +/- 7% of the Fas(+) CD3(+) T cells bound annexin V in NCs (P < 0.0001). In patients, Fas(+) CD3(+) cells preferentially underwent apoptosis and showed a loss of zeta chain expression. Significantly greater proportions of CD8(+) T cells than CD4(+) T cells were apoptotic (P < 0.0002), which indicates that CD8(+) T cells were especially sensitive to apoptosis. Serum levels of sFasL were lower in HNC patients with active disease than in NCs or in patients with no evident disease (P < 0.0183). This suggested utilization of sFasL produced in vivo and activation of the Fas/Fas ligand (FasL) pathway in Fas(+) T cells. Proportions of apoptotic T cells were higher in HNC patients than in NCs (P < 0.0001), and a subset of HNC patients with active disease had the highest proportions of circulating Fas(+) annexin V(+) T lymphocytes. The data indicate that the Fas/FasL pathway is involved in spontaneous apoptosis of circulating Fas(+) T lymphocytes in cancer patients. Fas/FasL interactions might lead to excessive turnover of T cells in the circulation and, consequently, to reduced immune competence in patients with HNC.