Mouth ulcers are commonly caused by infection but may be due to neutropenia. The most common form of hyper-IgM syndrome is of X-linked inheritance and caused by CD40 ligand gene mutations. Consider hyper-IgM syndrome in a male child with recurrent bacterial or opportunistic infections, neutropenia, hypogammaglobulinaemia (IgG and IgA) and normal T- and B-cell counts. In X-linked hyper-IgM syndrome: - the serum IgM concentration is normal in about 50% of cases. - transient or persistent neutropenia occurs in 70% of cases. First-line therapeutic options for hyper-IgM syndrome include regular intravenous immunoglobulin and prophylactic trimethoprimsulphamethoxazole.