Hydroxamic acid analog pan-histone deacetylase (HDAC) inhibitors (HA-HDIs) have shown preclinical and clinical activity against human acute leukemia. Here we describe HA-HDI-resistant human acute myeloid leukemia (AML) HL-60 (HL-60/LR) cells that are resistant to LAQ824, vorinostat, LBH589, and sodium butyrate. HL-60/LR cells show increased expression of HDACs 1, 2, and 4 but lack HDAC6 expression, with concomitant hyperacetylation of heat shock protein 90 (hsp90). Treatment with HA-HDI failed to further augment hsp90 acetylation, or increase the levels of p21 or reactive oxygen species (ROSs), in HL-60/LR versus HL-60 cells. Although cross-resistant to antileukemia agents (eg, cytarabine, etoposide, and TRAIL), HL-60/LR cells are collaterally sensitive to the hsp90 inhibitor 17-AAG. Treatment with 17-AAG did not induce hsp70 or deplete the hsp90 client proteins AKT and c-Raf. HL-60/LR versus HL-60 cells display a higher growth fraction and shorter doubling time, along with a shorter interval to generation of leukemia and survival in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Thus, resistance of AML cells to HA-HDIs is associated with loss of HDAC6, hyperacetylation of hsp90, aggressive leukemia phenotype, and collateral sensitivity to 17-AAG. These findings suggest that an hsp90 inhibitor-based antileukemia therapy may override de novo or acquired resistance of AML cells to HA-HDIs.