The t(12;21)(p13;q22) is a cryptic abnormality observed in 25% of children with B-lineage acute lymphoblastic leukemia (ALL), associated with a favorable prognosis. To determine whether specific cytogenetic abnormalities accompany the t(12;21), we analyzed the cytogenetic profiles of blast cells from 169 ALL cases positive for the t(12;21), previously identified by molecular methods. Only 13.6% of samples had normal karyotypes. Structural changes were detected in 89.7% of abnormal karyotypes, and numerical abnormalities in 47%. Rearrangements of 12p were the most frequent structural aberration (57 out of 146 patients with chromosomal abnormalities). Nonspecific deletions of chromosomes 6 and 9 were also found. The most frequent numerical abnormalities was trisomy for chromosomes 21. Blast cells were pseudodiploid (45.6%), hyperdiploid with 47 to 51 chromosomes (24.3%), hypodiploid with 44 to 45 chromosomes (10%), near-triploid (0.6%), or near-tetraploid (5.9%). Our results show that the t(12;21) is not associated with hyperdiploidy of 52 to 68 chromosomes or with the prognostic t(1;19), t(4;11) or t(9;22). Only children with B-lineage ALL who lack these abnormalities detected by conventional cytogenetics will probably benefit from additional testing by molecular methods to detect the t(12;21).