We report 12 cases of t(6;9)(p23;q34)-positive acute myeloid leukemia (AML), all classified using the criteria of the World Health Organization classification. There were 10 women and 2 men with a median age of 51 years (range, 20-76 years). Dysplasia was present in all cases (9 previously untreated), and basophilia was present in 6 (50%). Immunophenotypic studies showed that the blasts were positive for CD9, CD13, CD33, CD38, CD117, and HLA-DR in all cases assessed. CD34 was positive in 11 (92%) of 12, and terminal deoxynucleotidyl transferase was positive in 7 (64%) of 11 cases. The t(6;9) was the only cytogenetic abnormality detected in 7 cases (58%), and 5 cases had additional chromosomal abnormalities. Of 8 cases assessed, 7 (88%) had flt3 gene mutations. We conclude that t(6;9)-positive AML cases have distinctive morphologic features, an immunophenotype suggesting origin from an early hematopoietic progenitor cell, and a high frequency of flt3 gene mutations.