Complex chromosomal abnormalities are rare in adult patients with acute lymphoblastic leukemia (ALL). Using molecular methods, we characterized a complex karyotype involving chromosomes 3, 6, 11, 12, and 22 in a 38-year-old man with ALL. Cytogenetic analysis revealed the following karyotype: 46,XY,der(3)t(3;?6)(q22;?p21), - 6,add(11)(q23),add(12)(p13), + mar/46,XY. Because patients with 11q23 abnormalities have a poor prognosis and require aggressive treatment, we used fluorescence in situ hybridization (FISH) to fully characterize the abnormalities. FISH analysis showed no rearrangement of the MLL or ETV6-CBFA2 (TEL-AML1) genes; the wild-type ETV6 allele was deleted in most cells. The revised karyotype after the FISH analysis was as follows: 46,XY,der(3)t(3;12)(p13;p?13)del(3)(q21),der(6)inv(6)(p21q21)ins(6;3)(q21;q21q25),der(11)t (3; 11)(q25;q23),der(12)t(11; 12)(q23;p?12),t(12;22)(p13;q13). Although structural abnormalities involving 11q23 and 12p13 bands were identified by conventional cytogenetics, this report clearly demonstrates that molecular assays are needed to fully characterize gene rearrangements, complex translocations as well as to assign patients to the appropriate treatment group.