This study identifies multiple copies of the AML1 gene on a duplicated chromosome 21, dup (21), as a recurrent abnormality in acute lymphoblastic leukemia (ALL). Clusters of AML1 signals were visible at interphase by fluorescence in situ hybridization (FISH). In metaphase, they appeared tandemly duplicated on marker chromosomes of five distinct morphological types: large or small acrocentrics, metacentrics, submetacentrics or rings. The markers comprised only chromosome 21 material. Karyotypes were near-diploid and, besides dup (21), no other established chromosomal changes were observed. A total of 20 patients, 1.5 and< 0.5% among consecutive series of childhood and adult ALL respectively, showed this phenomenon. Their median age was 9 years, white cell counts were low and all had a pre-B/common immunophenotype. Although this series is not the first report of this abnormality, it is the largest, permitting a detailed description of the variety of morphological forms that duplicated chromosome 21 can assume.