In the present study, 8 flavonoids of three major flavonoid subgroups, namely flavones, flavonols and flavanones were tested for their cytotoxic and apoptogenic effects in human acute lymphoblastic leukemia MT-4 cells in vitro. Apoptotic cells were identified by DNA flow cytometric analysis. The effects of the flavonoids on the cell cycle patterns and activation of caspase-3 were also examined. Among the flavonoids tested, 7,8-benzoflavone, flavone, quercetin, chrysin, and galangine were shown to be effective apoptosis inducers. At concentrations corresponding to ED50, the flavonoids mentioned above exerted varying degrees of caspase-3 activation in MT-4 cells. The flavonoid-treated cells demonstrated different cell cycle profiles with accumulation in either G0/G1 (flavone, morin) or G2/M (7,8-benzoflavone, naringenin, quercetin, apigenin) phase. The induction of apoptosis did not correlate with phase-specific effects of flavonoid assayed. The relationship between chemical structure and apoptogenic activity of flavonoids tested is discussed.