Abstract: Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disorder characterised by microangiopathic haemolytic anaemia, thrombocytopenia and signs of ischaemic organ dysfunction such as neurological or renal impairment and fever. The diagnosis of TTP should be considered in any child presenting with thrombocytopenia, in particular those children with microangiopathic haemolysis, atypical immune thrombocytopenia purpura or Evan’s syndrome. Distinguishing TTP from haemolytic uraemic syndrome is difficult, but where there is doubt about the diagnosis, a presumptive diagnosis of TTP should be made to allow potentially life-saving therapy with therapeutic plasma exchange. Recent advances in the molecular basis of the disease have resulted in assays for ADAMTS-13 activity, inhibitor levels and ADAMTS-13 mutation analysis. These assays help to distinguish TTP from haemolytic uraemic syndrome. However, the performance characteristics of these assays in the diagnosis and management of TTP are yet to be defined.