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A male child presented with recurrent respiratory infections, otitis media, and oral ulceration and was found to be neutropenic. Investigations showed hypogammaglobulinaemia with normal serum IgM and a novel deletion in the gene for CD40 ligand on his X chromosome. Intravenous gammaglobulin did not lead to resolution ofhis neutropenia; G-CSF was also necessary. (Arch Dis Child 1996; 74: 458-459) Keywords: hyper IgM syndrome, neutropenia, CD40 ligand deficiency.The hyper IgM syndrome is characterised by low plasma concentrations of IgG, IgA, and IgE associated with normal or increased levels of IgM. It is usually an X linked primary immunodeficiency caused by a mutation in the gene for the T cell surface protein CD40 ligand' which is involved in T cell interaction with B cells and the induction of isotype switching. The condition often presents in the first year of life with recurrent respiratory tract infections and with opportunistic infection, especially with P carinii. In vitro assays of T cell number and proliferation are normal. Neutropenia is occasionally associated with the syndrome but the reason for this is unclear. Treatment with prophylactic cotrimoxazole and intravenous gammaglobulin prevents infections and is reported to alleviate the neutropenia.