Natural killer cell activity was assessed in 100 previously untreated pharyngeal carcinoma patients. Diminished natural killer cell function in these patients was associated with an increased risk of death from uncontrolled regional and distant metastases. During the assessment, the cell line MDA686-Ln was established from a metastatic pharyngeal carcinoma of a patient with low natural killer cell cytotoxicity. The initially cytotoxicity-resistant cell line could be lysed when natural killer cell cytotoxicity was enhanced in vitro either through enrichment of a Leu 19+ natural killer cell population by fluorescent-activated cell sorting or by interleukin-2 activation. Additionally, increased circulating immune complexes were identified in these patients, subsequently isolated, and found to block natural killer cell reactivity against MDA686-Ln. In light of this negative interaction, 38 patients were randomly evaluated for both circulating immune complex levels and natural killer cell function. Both parameters examined together were complementary in defining the risk of death with disease; four of five deaths occurred in patients with both high circulating immune complex levels and low natural killer cell function. Results support the biologic modification of natural killer cell activity for controlling metastatic pharyngeal carcinoma and point to the potential confounding influence of circulating immune complex.