Clinical manifestations suggesting WAS/XLT are often present at birth and consist of petechiae, bruising, and bloody diarrhoea. Excessive haemorrhage after circumcision is an early diagnostic clue. Infections, including purulent otitis media, pneumonia and skin infections, are common during the first 6 months of life. Infections are most often caused by bacteria and viruses such as CMV, rarely by Pneumocystis carinii. Patients with XLT have fewer problems with eczema and infections and are often misdiagnosed as having idiopathic thrombocytopaenia (ITP). Thrombocytopaenia associated with small platelet volume is a consistent finding in both classical WAS and XLT and is a key diagnostic indicator. In most patients the mean platelet volume is half that of normal control subjects. Life-threatening bleeding, including severe oral bleeding, gastrointestinal bleeding, and intracranial haemorrhage, has been reported in up to 30% of patients. Compromised humoral and cellular adaptive immunity is a hallmark of classical WAS. Common findings include mild to moderate lymphopaenia, defective T cell proliferation in response to TCR stimulation, low levels of IgM and high levels of IgA and IgE. Antigenspecific T and B cell responses, particularly to polysaccharides, are also impaired in patients with WAS. Patients with XLT by definition have minimal immunological disturbances. Autoimmune diseases are frequent, the most common being haemolytic anaemia, thrombocytopaenia, neutropaenia followed by vasculitis, renal disease, Henoch-Schonlein-like purpura, and inflammatory bowel disease. Incidence of autoimmune disease is less in XLT but has been reported to occur. Prominent features of XLN include chronic neutropaenia and monocytopaenia, although some cases have values that fall within the low-normal range. Low-normal IgA levels, low to low-normal platelet counts (normal mean platelet volume) and reduced natural killer (NK)-cell counts have been reported.