Immunoglobulin (Ig) A deficiency has long been recognized in patients with chromosome 18 abnormalities. We present the case of a young girl in whom a chromosome 18p deletion syndrome (46,XX,del[18][p11.1]) was associated not only with IgA deficiency, but also with an inability to make antibody to the unconjugated pneumococcal polysaccharide vaccine, Pneumovax II, indicating a concomitant specif...