The present invention relates to methods of treating cancers, e.g., leukemia. More specifically, the present invention relates to methods of treating acute and chronic leukemias including Acute Lymphocytic Leukemia (ALL), Acute Myeloid Leukemia (AML), Chronic Lymphocytic leukemia (CLL), Chronic myeloid leukemia (CML) and Hairy Cell Leukemia, by administration of pharmaceutical compositions comprising HDAC inhibitors, e.g., suberoylanilide hydroxamic acid (SAHA). The oral formulations of the pharmaceutical compositions have favorable pharmacokinetic profiles such as high bioavailability and surprisingly give rise to high blood levels of the active compounds over an extended period of time. The present invention further provides a safe, daily dosing regimen of these pharmaceutical compositions, which is easy to follow, and which results in a therapeutically effective amount of the HDAC inhibitors in vivo.