We report the results of immunologic studies in a family in which the father (III-5) and his two daughters (IV-7 and IV-8) had the hyper-IgM syndrome (IHIS). Repeated immunoglobulin levels done on III-5 showed a typical IHIS pattern: low IgG, traces of IgA, and high IgM. IV-7, who also had stage IIA Hodgkin's disease, had a similar pattern except after irradiation therapy to sites of disease, when IgM dropped to normal range while IgG and IgA remained low. IV-8, on the other hand, had normal IgG and IgA and moderately elevated IgM until age 18 months, when she gradually developed the IHIS pattern. All three patients had normal numbers of B cells (sIg) and of T cells, although IV-7 had increased suppression. Finally, all three patients shared the A3,B7 haplotype and none was blood type O. IHIS is not necessarily X linked, is not associated with blood type O, and appears to be heterogeneous even within the same family. Inheritance in this family is apparently autosomal dominant and the father may represent a new mutation.