Cartilage hair hypoplasia (CHH) is an autosomal recessive form of short-limbed dwarfism prevalent among the Old Order Amish. Mild to moderately severe cellular immunodeficiency is associated with this disorder. Antibody synthesis is, however, normal in CHH. Individuals affected with CHH were found to have marked impairment of T-lymphocyte function due to an intrinsic defect in cell proliferation. Defective proliferation was also found in B cells and fibroblasts from CHH individuals suggesting that impaired T-cell function reflects a generalized defect in cell proliferation in this syndrome. Studies of cytotoxic mechanisms in CHH patients revealed that proliferation-dependent mechanisms (e.g., cell-mediated cytotoxicity and natural killer [NK]-like activity) were markedly impaired while proliferation-independent NK activity was normal. In spite of impairment of T-cell function, an increased incidence of malignancy was not observed in CHH patients. These observations suggest that NK activity is vital in host defense against malignancies and that marked impairment of T-cell-mediated immunity need not be associated with an increased susceptibility to malignancy if NK function is preserved.