We report on two sisters affected by congenital alopecia, nail dystrophy, and a severe T‐cell immunodeficiency, presumably inherited as an autosomal‐recessive disorder. The T‐cell defect was characterized by severe functional impairment, as shown by the lack of proliferative response and upregulation of activation markers following mitogen stimulation. The functional abnormality occurred in spite of the presence of phenotypically mature T cells, thus suggesting the qualitative nature of the defect. This is the first observation reported on an ectodermal disorder, characterized by alopecia and nail dystrophy, observed at birth, in association with a primary immunodeficiency. The hypothesis that these two events may be causally related is discussed. © 1996 Wiley‐Liss, Inc.