Thrombosis and haemorrhage are major causes of morbidity in patients with myeloproliferative diseases. However, in chronic myeloid leukaemia (CML) these complications are less likely to occur. Therefore aggressive lowering of the platelet count has not been recommended in asymptomatic patients with CML. We describe 2 patients, 1 with acute myeloid leukaemia in remission and the other with CML in accelerated phase, in whom severe intracranial thrombotic complications unexpectedly developed: blindness in the former and brain stem damage with deep unconsciousness in the latter. During already ongoing cytostatic treatment both of them recovered dramatically, which seemed to have a temporal relationship to the reduction of their platelet counts. We consider that the thrombotic events in these patients were due to their high platelet counts and recommend treatment for symptomatic thrombocytosis.