A 22-year-old male was diagnosed as having immunodeficiency with hyper-IgM based upon recurrent pneumonia, marked elevation of serum IgM and markedly decreased level of IgG. IgG-or IgA-bearing B cells were not detected in peripheral blood while a number and a proportion of peripheral blood T lymphocytes were normal. Peripheral blood lymphocytes from this patient proliferated normally in response to T-independent and T-dependent B cell mitogens, and to T cell mitogens. Furthermore, the same type of dysgammaglobulinemia with increased IgM was found in the patient's father and brother. From these observations, it is suggested that it is a rare case of autosomal dominant or polygenal inheritance of hyper-IgM immunodeficiency.