A review of the cytogenetic analyses of a cohort of unselected patients with acute lymphoblastic leukemia from a single institution (1981-1989) confirmed hyperdiploidy > 50 as a favorable prognostic feature and pseudodiploidy, especially with chromosomal rearrangements, as an adverse one. In the early years of the study, a high incidence of diploid karyotype was identified, but with time (better banding and more intensive search) the incidence has fallen to 7.7%. Many such patients have been shown to be pseudodiploid. Molecular genetic techniques are increasingly identifying clonal abnormalities.