X-linked hyper IgM syndrome (HIGM1), a disorder of abnormal T- and B-cell function, is characterized by low serum concentrations of IgG, IgA, and IgE with normal or elevated serum concentrations of IgM. Mitogen proliferation may be normal, but NK- and T-cell cytotoxicity can be impaired. Antigen-specific responses are usually decreased or absent. Total numbers of B cells are normal but there is a marked reduction of class-switched memory B cells. Defective oxidative burst of both neutrophils and macrophages has been reported. The range of clinical findings varies, even within the same family. More than 50% of males with HIGM1 develop symptoms by age one year, and more than 90% are symptomatic by age four years. HIGM1 usually presents in infancy with recurrent upper- and lower-respiratory tract bacterial infections, opportunistic infections including Pneumocystis jirovecii pneumonia, and recurrent or protracted diarrhea that can be infectious or noninfectious and is associated with failure to thrive. Neutropenia is common; thrombocytopenia and anemia are less commonly seen. Autoimmune and/or inflammatory disorders (such as sclerosing cholangitis) as well as increased risk for neoplasms have been reported as medical complications of this disorder. Significant neurologic complications, often the result of a CNS infection, are seen in 5%-15% of affected males. Liver disease, a serious complication of HIGM1 once observed in more than 80% of affected males by age 20 years, may be decreasing with adequate screening and treatment of Cryptosporidium infection.