Trisomy 4 is an uncommon numerical chromosomal aberration in acute leukemia. We describe three cases of trisomy 4, occurring in two patients with acute myeloid leukemia (M1 and M5a) and in one patient with T-cell acute lymphoblastic leukemia. Our results suggest that trisomy 4 may occur in a broader range of hematologic and malignancies than previously described.