Summary. The Platelet Function Analyzer (PFA-100®) was used to measure platelet function in paediatric patients with destructive versus underproduction thrombocytopenia. Closure time (CT) and total volume (TV) measurements with standard 150 μm apertures discriminated between patients with similar platelet counts from 30 to 150 × 109/l. However, at platelet counts < 30 × 109/l, a 100-μm aperture (experimental) gave the best assessment of platelet function. TV results could be analysed even when CTs were indeterminate. Further investigations are warranted to more fully understand the relationships among platelet function as measured by the PFA-100® in standard/experimental modes, bleeding and transfusion outcome in thrombocytopenia.