Tetraploid karyotypes without structural chromosome abnormalities were found in approximately 50% of the bone marrow cells in two patients with acute lymphoblastic leukemia with L2 morphology and "null cell" immunophenotype. Strict tetraploidy (4n = 92) has not been reported as the sole karyotypic rearrangement in bone marrow neoplasia, but may represent a previously unrecognized cytogenetic leukemia subtype.