A patient with acute lymphocytic leukemia was found to have a hyperdiploid karyotype, characterized by hexasomy 21 and del(7)(p15). It was shown that the four extra copies of chromosome 21 were all derived from only one of the homologous chromosomes. Molecular analysis showed that the patient had a deletion of both alleles of the retinoblastoma (RBI) gene. These results suggest that multiple events, including loss of RBI gene function and amplification of a key gene(s) on chromosome 21, have contributed to the leukemic transformation.