The genetic transmission of X-linked agammaglobulinaemia (XLA) can be determined with high probability using closely linked DNA restriction fragment length polymorphisms (RFLP's). In a family known to be at risk for XLA in male offspring, RFLP analysis demonstrated that the mother was an XLA carrier and her newborn son was affected. The infant developed immunological deficiencies a few months later, confirming the diagnosis. RFLP analysis provides a method for carrier detection, prenatal diagnosis and presymptomatic diagnosis of XLA, which plays a significant role in prevention of the disease.