The PFA-100® device is a new instrument for the in-vitro testing of platelet function. Primary haemostasis is stimulated by recording the closure time taken for platelets to seal a 150 μm aperture in the centre of a membrane coated with collagen and either epinephrine or ADP. Patients with type 3 von Willebrand's disease (n= 4) all had infinitely prolonged closure times (> 200 s) with both types of cartridge. A patient with afibrinogenemia exhibited only slightly prolonged closure times of 111 and 166 s for the ADP and epinephrine membranes, respectively. Patients with Glanzmann's thrombasthenia (n= 6) and Bernard Soulier syndrome (n= 2) had grossly prolonged closure times (> 200 s) with both types of cartridges. These results confirmed that the PFA-100® system was highly dependent on normal von Willebrand factor, glycoprotein lb and glycoprotein IIb/IIIa levels but not on plasma fibrinogen. Patients with storage pool disease (n= 6) and Hermansky Pudlak syndrome (n= 7) had prolonged closure times with the epinephrine cartridge. There was no evidence of enhanced platelet function in patients with antiphospholipid syndrome, in sickle-cell disease or thalassemia. However, ingestion of aspirin resulted in a near consistent and significant prolongation of the closure time for the epinephrine cartridge but not for the ADP cartridge in both normal subjects and patients. The test offers a reliable, reproducible, rapid and simple means of assessing high-shear platelet function in vitro.