Twenty new cases of acute lymphoblastic leukemia (ALL) with the dicentric chromosome dic(9;20)(p1113;q11) are presented. This chromosomal abnormality is difficult to identify from G-banding alone. It masquerades as monosomy 20 and is only accurately identified by fluorescence in situ hybridization (FISH). Monosomy 20 was found in 59/2790 patients with successful karyotypes entered to the Leukaemia Research Fund/UK Cancer Cytogenetics Group Karyotype Database in ALL (LRF/UKCCG Karyotype Database). FISH revealed dic(9;20) in 20/25 cases with available material. Extra copies of chromosome 21 were found in 8 of the 20 cases. Patients were 14 females and six males, aged 1-32 years (median 4 years), with leukocyte counts of 2-536 (median 23) x 109/l and immunophenotypes of common or pre-B ALL (17 cases), T-ALL (one case) or unknown (two cases). Four patients relapsed at 2, 22, 28 and 47 months and two died at 49 and 63 months (median follow-up 37 months). FISH studies on the remaining five patients showed one with monosomy 20 and four with other rearrangements of the chromosome. This study has increased the number of reported cases of dic(9;20) from 17 to 37. It has identified dic(9;20) in one case of T-ALL and shows an association of this translocation with trisomy 21.