Fourteen cases of dic(9;12)(p11-13;p11-12) in early B-lineage acute lymphoblastic leukemia (ALL) and other hematological malignancies are reported with a review of the literature. Altogether 36 cases were collected for analysis: ALL at diagnosis (31 cases) or in relapse (one case), chronic myeloid leukemia in lymphoid blast crisis (two cases), T-cell lymphoblastic lymphoma (one case) and T-cell non-Hodgkin's lymphoma (one case). We report the first cases of dic(9;12) with a T-cell phenotype. Dic(9;12) occurs predominantly in B-progenitor ALL of childhood and young adults (age range, 1-47 years, median 12 years) but not of infancy. One or more adverse clinical features, age > 10 years, WBC > 100 x 10(9)/l, pre-B immunophenotype, platelets < 100 x 10(9)/l, were found in over 90% of cases. Additional structural chromosomal changes or trisomy 8 were frequently present. Nevertheless with a median follow-up of 5 years, 29/31 cases (94%) remain in first remission conferring an excellent prognosis to this leukemia. Additional cases are being sought to confirm the prognostic value of this cytogenetic aberration in various hematological malignancies.