A 27-year-old male who visited our hospital because of pneumonia was diagnosed as hyperIgM immunodeficiency syndrome. His serum IgM level was markedly elevated, while the serum level ofIgD was normal with a markedly decreased level of serum IgG and IgA. The proportion of T and B cells of peripheral blood lymphocytes was normal. However, B cells bearing surface IgG or IgA were not detectable by immunofluorescence technique. There was a consanguineous marriage in his family, suggesting that his disorder was caused by a genetic abnormality such as X-linked recessive and also autosomal recessive inheritance, although further study is necessary. CD40ligand CDNA did not appear to contain any abnormal changes within the coding region. (Internal Medicine 34: 640-642, 1995)