Three patients with acute myeloblastic leukaemia and blast cell counts greater than 100 X 10(9)/1 (100 000/mm3) died unexpectedly soon after blood transfusion. In two cases postmortem examination disclosed cerebral leukostasis. Analysis of the records from the MRC's fourth and fifth acute myeloid leukaemia trials showed that in the first week after diagnosis mortality was five times greater in patients with blast counts above 100 X 10(9)/1 than in patients with lower counts. Age and platelet count did not explain this excess. The mean haemoglobin concentration in the patients with high blast counts who died within the first week was 10.5 +/- 2.8 g/dl, which was significantly higher than that in the surviving group (7.6 +/- 2.4 g/dl). Only half the patients received chemotherapy within two days of diagnosis. Leukostasis is an important cause of early death in patients with high blast counts, and the increase in viscosity produced by transfusing to a haemoglobin concentration above 10 g/dl may lead to sudden deterioration. Transfusion to such concentrations should be avoided until the blast count has been reduced by early chemotherapy.