Selective absence of serum and secretory IgA is probably the most common form of human immunodeficiency. High frequencies of recurrent sinusitis, otitis media, pneumonia, and atopy were noted among a group of 75 such patients, all but 4 of whom were Caucasian. Seven instances of familial absence of IgA were detected among 106 relatives of 34 of the group; in 1 family 1 member from each of 3 successive generations was affected. Two IgA-deficient children were later found to have normal amounts of serum IgA. Despite their humoral deficit, B lymphocytes bearing surface IgA were detected in 9/9 IgA-deficient patients in immunofluorescence studies of their peripheral blood lymphocytes. Although in vitro lymphocyte responses to 2 putative T-cell mitogens and to allogenic cells were normal, results of spontaneous rosette formation studies with sheep erythrocytes raise the possibility of a lymphocyte subpopulation deficit in this condition.