Gardner's syndrome is an autosomal dominant disease presenting multiple colonic polyps with a predisposition for malignant change. In addition to colonic polyp formation by early adolescence, extracolonic lesions appear often prior to polyp formation. One theoretical mechanism for the origin of polyps and malignancies in Gardner's syndrome is a genetic defect in the natural killer cell activity of patients with this disease. Natural killer cells are a of lymphocytes that spontaneously lyse tumor cells subpopulation and virally transformed cells. A study was undertaken to determine the natural killer activity of patients with Gardner's syndrome .