Two siblings with pansinusitis, nasal polyps, and bronchiectasis were found to have histocompatibility lymphocyte antigen (HLA) class I antigen deficiency ("bare lymphocyte syndrome") and dysfunction of natural killer cells. Reduced class I cell surface expression resulted from a single mutation in the TAP2 gene, which is located in the class II region of the major histocompatibility complex and encodes subunit 2 of the class I peptide transporter. The defect was transmitted in an autosomal recessive manner. This deficiency did not lead to severe viral infections but was apparently associated with susceptibility to bacterial infections of the respiratory mucosae. We suggest that class I HLA typing should be systematically performed in children with unexplained bronchiectasis.