In the blast cells of children with acute lymphoblastic leukemia (ALL) more than 50 chromosomes can be observed in a quarter of cases. As a rule these children have a good prognosis. However, some of these patients develop a relapse of their basic disease. There is only poor information about the significance of distinct additional chromosomes for the prognosis. The white blood cell count (WBC) at the time of diagnosis is a further very important prognostic factor in childhood ALL. Therefore we compared the relation between trisomy of distinct chromosomes and the initial white blood cell count of 41 children with common ALL and hyperdiploid karyotype. The modal chromosome number ranged from 50 to 60 chromosomes. Most frequently, the chromosomes X, 4, 6, 8, 10, 17, 18 and 21 were multiplied. Additionally, in 25 of the 41 cases structural chromosome aberrations were observed. The average WBC was estimated as 9.6 Gpt/l with a range from 1.8 to 41.5 Gpt/l. The initial WBC was slightly increased in patients with the additional chromosome X, 6, 11, 12 or 19 and distinctly decreased in children with the additional chromosome 8 or 9 in their hyperdiploid blast cells. No patient with an additional Chromosome 9 showed a WBC higher than 10 Gpt/l and only 2 out of the 12 children with an additional chromosome 8 revealed an initial WBC higher than 10 Gpt/l. Additional structural chromosome aberrations were without influence on the WBC.