From literature data on 3,149 cytogenetically abnormal cases of acute lymphoblastic leukemia (ALL) 1,118 clones with a gain of 1-11 chromosomes were retrieved. Within each subgroup of polysomy, the distribution of gained chromosomes was clearly nonrandom. In general, two different patterns were seen in ALLs with 1-5 and 6-11 extra chromosomes, respectively. In the former group, chromosomes X, 8, and 21 were consistently over-represented, and in the latter most or all polysomy subgroups contained an excess of chromosomes X, 4, 6, 10, 14, 17, 18, and 21. Chromosomes Y, 1-3, 5, 7, 9, 11-13, 15, 16, 19, and 20 were never significantly over-represented in any polysomy subgroup. The results of the present study confirm the nonrandom nature of chromosomal gain in ALL and suggest that when ALLs are to be subdivided into those with moderate and pronounced hyperdiploidy, the former should contain cases with 47-51 chromosomes and the latter those with 52-57 chromosomes.