Chronic lymphocytic leukemia (CLL) develops due to an imbalance between apoptosis and proliferation of B lymphocytes. Chrysin induced apoptosis in leukemia cell lines such as U937, MO7e, THP-1 and HL-60, but there has not yet been data demonstrating the apoptotic effect of chrysin on CLL cells. Therefore, in our investigation we examined the cytotoxicity of chrysin against two leukemia cell lines, MOLT-4 and JVM-13, peripheral blood lymphocytes isolated from B-CLL patients and peripheral blood mononuclear cells (PBMC) from healthy individuals in vitro. The effect of chrysin on viability of MOLT-4 and JVM-13 cell lines, B-CLL cells derived from 28 patients and PBMC from 16 healthy subjects was determined by MTT assay. The type of cell death induced by chrysin was verified by Annexin V/7AAD assay and acridine orange and ethidium bromide (AO/EB) staining assay. Intracellular localisation and endogenic expression of apoptotic proteins including Bax, Bcl-2, cytochrome c and caspase-3 were determined by flow cytometry and fluorescent microscopy. Our results demonstrated that exposure of MOLT-4, JVM-13 cell lines and B-CLL cells to the concentration of chrysin of 10μM and higher selectively decreased viability of cells in this cell population, but not in the PBMC derived from healthy subjects; LC50 values of chrysin for B-CLL cells were 51μM for 24 hours and 32μM for 48 hours of incubation, respectively. Our findings demonstrated that chrysin induces the activation of proapoptotic Bax and decreases the expression of antiapoptotic Bcl-2 protein, releases cytochrome c from mitochondria into cytosol and cleavages/activates caspase-3, subsequently leading to the activation of apoptosis of B-CLL cells. Together, these findings suggest that chrysin selectively induces apoptosis of peripheral blood lymphocytes isolated from human chronic lymphocytic leukemia patients via mitochondrial pathway in vitro and that it might have a promising role as a potential future antileukemic remedy.