Thrombocytopenia in adult patients is usually acquired. Important causes are medication, pregnancy, infections and auto-immune disorders. Here we present 2 patients, a 32-year-old and a 45-year-old man, in whom congenital thrombocytopenia was discovered in adulthood, caused by a mutation in the gene for Wiskott-Aldrich syndrome. This congenital thrombocytopenia appears in different forms. Indications for the disorder are chronic thrombocytopenia, a family history and additional clinical signs such as eczema and renal insufficiency. Measurement of the thrombocyte size is a helpful tool in the diagnosis. Recognition of congenital thrombocytopenia can have important clinical consequences.