In order to determine the significance of cell size together with high leukocyte count (>30x10(9)/L) in acute myeloblastic leukemia (AML), we evaluated the percentages of small, medium and large cells in 33 children with AML. All of the 10 patients with a high leukocyte count and 14 of the 23 patients with a low leukocyte count (<30x10(9)/L) died or experienced a relapse within the first year. The mean small cell percentage of patients with high leukocyte counts was significantly lower than that of patients with low leukocyte counts (p<0.05). The percentages of small, medium and large cells of patients with high leukocyte counts and of patients with low leukocyte counts who died or experienced a relapse within the first year were similar. The percentage of medium cells of patients with high leukocyte counts was significantly higher than that of surviving patients with low leukocyte counts (p<0.05). The mean percentages of small, medium and large cells were similar in patients who died or experienced a relapse and surviving patients with low leukocyte count. We conclude that cell size has prognostic significance when the leukocyte count at admission is over 30x10(9)/L, although confirmation seems necessary with a larger population of patients.