A cell line has been established from blood lymphoblasts of a female patient with acute lymphoblastic leukemia (ALL) shown to have near-haploid (27 chromosomes) cells in the bone marrow. The findings about the cell line were: 1) The frequency of near-haploid cells in culture decreased with time from 98.2% when the culture was started to 5.4% 15 months later. 2) Most of the other cells except the near-haploid ones were hyperdiploid, i.e., duplicates of the cells with near-haploid chromosome constitutions. 3) Chromosome evolution was seen in the near-haploid clones. The possible ancestor clone (clone A) had 27 chromosomes, one of each pair except #10, 14, 18, and 21, which were disomic. A suggested evolution process is: clone A→clone B (26 chromosomes: clone A, −#10)→clone C (27 chromosomes: clone B, +X)→clone D (26 chromosomes: clone C, −#21), clone E (28 chromosomes: clone C, +#20). Clones B and D, each with 26 chromosomes, appeared to contain the lowest number of chromosomes ever described for human somatic cell clones in vitro. 4) Changes in the constitutions of the hyperdiploid cell clones were preceded by evolution and changes in the near-haploid clones. 5) In near-haploid cells with 2 X-chromosomes, 1 exhibited late DNA replication; in hyperdiploid cells with 3–5 X-chromosomes, 2 were non-late DNA-replicating. 6) Fresh (uncultured) and cultured leukemia cells were antigenically typical non-T, non-B or common type ALL cells (positive for lalike and null-type ALL antigens and negative for surface membrane immunoglobulin).