The t(3;21)(q26.2;q22) translocation is rare in cases of myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). We studied 17 patients with MDS/AML associated with t(3;21) and compared them with 17 patients with MDS associated with inv(3) (q21q26.2)/t(3;3)(q21;q26.2), because these entities share 3q26 locus abnormalities. The t(3;21) group included 9 men and 8 women, with a median age of 62 years (range, 13-81 years). One case was de novo AML and 16 cases were therapy-related, including 12 MDS (blasts, <15%) and 4 AML (blasts, 33%-50%). All patients had multilineage dysplasia, whereas none had thrombocytosis. Additional cytogenetic aberrations were identified in 12 cases, including -7/7q (n = 9) and a complex karyotype (n = 7). All patients died, with 1- and 2-year survival rates of 35% and 6%, respectively. Although multilineage dysplasia and frequent association with -7/7q were similar in both groups, MDS/AML cases associated with t(3;21) have a higher frequency of therapy-related disease and shorter survival times, suggesting that they are distinct from MDS/AML cases associated with inv(3)/t(3;3).