Seventeen cytogenetic laboratories in eight European countries contributed karyotypic, hematological, clinical and follow-up data from 550 patients with an acquired abnormality of 11q23. The patients had acute lymphoblastic leukemia (254), acute myeloid leukemia (250), unspecified, undifferentiated, biphenotypic acute leukemia or myeloproliferative disorder (18 cases together), or myelodysplastic syndrome (MDS) (28). The patients were classified by cytogenetic subgroup as t(4;11) (183 cases), t(6;11) (30) cases), t(9;11) (125 cases), t(10;11) (20 cases), t(11;19) (53 cases), ‘other’ abnormalities of 11q23 (82 cases) and del(11)(q23) (57 cases). Manuscripts were prepared on each cytogenetic subgroup, on MDS, on secondary hematological malignancies (40 cases) and on 11q23-translocation derivatives. For each subgroup the following aspects were investigated: associated clinical features, additional karyotypic change, distribution between hematological subtypes and between different age groups, prognosis at different age groups, and the impact of bone marrow transplantation on survival. The Workshop confirmed some previous findings from smaller studies, challenged others, identified new chromosomal partners and threw new light on less well documented aspects of 11q23 malignancies. The large number of cases investigated in a coordinated manner gives authoritative support to the findings. The Workshop thus demonstrates the value of collaborative European studies in the cytogenetics of malignancy.