Typical X linked agammaglobulinaemia (XLA) is characterised by absence of immunoglobulin production and lack of mature B cells. The gene responsible for XLA has recently been identified, and codes for a B cell tyrosine kinase, BTK. A family affected by a B cell immunodeficiency, which is less severe than classical XLA, is described but they had a pedigree suggestive of X linked inheritance. Demonstration of a mutation in the BTK gene confirms that this is a mild form of XLA.