A 52-year-old man developed an illness which lasted for 2 years until its fatal termination. It comprised in chronological order: mild asthma, skin irritation, mature eosinophilia in the peripheral blood, and finally an encephalopathy. The diagnostic problem of distinguishing between eosinophilic leukaemia and an eosinophilic leukaemoid reaction is discussed. The cause of the encephalopathy was discovered at autopsy to be a widespread intravascular thrombotic process. The question of future possible therapy is discussed.