Two patients with acute leukemia were found to have trisomy 11 as the sole chromosomal abnormality. This karyotype has previously been reported in adults with the myelodysplastic syndrome or acute nonlymphocytic leukemia (ANLL). In contrast to previously described cases, both of these patients were young and female. One patient with ANLL had sequential karyotype analysis with trisomy 11 both at presentation and relapse with apparent loss during remission. The other patient had acute lymphoblastic leukemia (ALL) both at presentation and at relapse, although the karyotype was only obtained at relapse. This is the first report of trisomy 11 in ALL.