Heat shock protein (HSP) 70 is aberrantly expressed in different malignancies and has emerged as a promising new target for anticancer therapy. Here, we analyzed the in vitro antileukemic effects of pifithrin-μ (PFT-μ), an inhibitor of inducible HSP70, in acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) cell lines, as well as in primary AML blasts. PFT-μ significantly inhibited cell viability at low micromolar concentrations in all cell lines tested, with IC50 values ranging from 2.5 to 12.7 μ, and was highly active in primary AML blasts with a median IC50 of 8.9 μ (range 5.7-37.2). Importantly, higher IC50 values were seen in normal hematopoietic cells. In AML and ALL, PFT-μ induced apoptosis and cell cycle arrest in a dose-dependent fashion. PFT-μ also led to an increase of the active form of caspase-3 and reduced the intracellular concentrations of AKT and ERK1/2 in NALM-6 cells. Moreover, PFT-μ enhanced cytotoxicity of cytarabine, 17-(allylamino)-17-desmethoxygeldanamycin, suberoylanilide hydroxamic acid, and sorafenib in NALM-6, TOM-1 and KG-1a cells. This is the first study demonstrating significant antileukemic effects of the HSP70 inhibitor PFT-μ, alone and in combination with different antineoplastic drugs in both AML and ALL. Our results suggest a potential therapeutic role for PFT-μ in acute leukemias.