Eight patients with Hyper-IgM syndrome were subjected to clinical and immunological evaluation. There were seven males and one female. All the patients had recurrent pyogenic infections; one had lymphoid hyperplasia with centrally necrotic granulomas, and one had gingivitis with neutropenia. Isohemagglutinin titers were either high or normal in all the patients and five had group 0 blood. The percentage of IgM-bearing cells were normal in five patients. The percentage of T cells were normal in all the patients, helper T cells were decreased in two patients, and suppressor T cells were increased in four patients. These results suggest that at least in some patients, the imbalances of T cell subsets may play a role in the pathogenesis of the disease rather than it being attributed to an intrinsic B cell defect.