Dicentric (9;20) [dic(9;20)] is a rare recurring chromosome abnormality in leukemia patients. In this study, we report three adult patients with acute lymphoblastic leukemia (ALL) with dic(9;20) anomaly. All three patients were diagnosed as cluster of differentiation 10 (CD10) positive B cell ALL, achieved complete remission after induction chemotherapy, and died of leukemia relapse within 1 year after diagnosis. Specimens for chromosome analysis were prepared by direct method and/or short-time culture of bone marrow cells. Karyotyping was performed by R-banding technique. Dual-color fluorescence in situ hybridization (FISH) was performed using chromosome 9 and chromosome 20-specific centromeric probes. Karyotype analysis showed that all three patients had dic(9;20)(p11-13;q11), in which the dicentric nature of the derived chromosome was confirmed by interphase FISH. Dicentric (9;20)(p11-13;q11) may be specifically associated with ALL, and monosomy 20 may be a pointer to dic(9;20) in ALL. The prognostic significance of dic(9;20) in adult patients with ALL remains to be determined.