Peripheral blood mononuclear cells (PBMC) from patients with active HPV16-associated pre-malignant and malignant anogenital lesions display a significantly decreased NK-cell activity against HPV16-harboring SKv keratinocytes (NK/SKv) while their cytotoxicity against erythroleukemic K562 cells (NK/K562) remains unaffected. A similar defect can also be seen in some healthy individuals displaying no symptoms of HPV infection (low responders). Analysis with specific Leu IIa monoclonal antibodies (MAbs) has revealed that all patients as well as weakly responding control subjects had normal numbers of circulating CD16+ NK cells. However, PBMC from patients with active disease and weakly responding controls displayed a significantly decreased ability to bind SKv cells. Binding of K562 was in the normal range. In patients in whom the lesions were successfully removed or regressed spontaneously (patients with no lesions), NK/SKv activity did not differ from that of normally responding healthy subjects and the ability of their PBMC to bind SKv cells was unaffected. To determine whether an abrogated NK/SKv cytotoxicity may be corrected by NK-cell stimulatory cytokines. PBMC were pre-incubated overnight with IL-2 and interferon-alpha. Both cytokines stimulated NK/K562 activity in all tested groups. Significant stimulation of NK/SKv activity was observed in PBMC from normal and weakly responding controls as well as patients with no lesions. No increase could be seen in patients with active disease. Evaluations of NK-cell activity before and after surgical removal or spontaneous regression of the lesions showed normalization of primarily depressed NK/SKv activity. Malignant progression was associated with a significant drop in SKv cell killing. Our results suggest that abrogation of NK-cell activity against HPV16-harboring targets in patients with HPV16-associated anogenital neoplasia is associated with restricted inability to recognize the disease-specific target cells, and may depend on persistence of the lesions.