Background: One of the major paths for drug development is the study of bioactivities of natural products. Therefore, the aim of this study was to compare the cytotoxic effects of aqueous extract of whole Cuscuta chinensis Lam., which is a traditional medicinal herb commonly used in Iran and other oriental countries, on the human caucasian acute lymphoblastic leukemia (CCRF-CEM) and another human lymphocyte, Jurkat (JM) cell lines. Methods: In vitro cytotoxic screening with various concentrations (0, 0.1, 1, 10, 25 and 50 µg/ml) of the extract was performed using microscope and methyl tetrazolium bromide test (MTT). Results: The minimum effective concentration of the plant extract was 1 µg/ml, and increasing the dose to 10 µg/ml induced increasingly stronger effects. The inhibitory concentration 50% (IC50) of the extract against CCRF was about 3 µg/ml in 24 hours and 2.5 µg/ml in 48 hrs. In contrast, the extract did not have cytotoxic effect for the JM cells at these doses. Conclusion: The findings of the present study suggest that C. chinensis is toxic against CCRF-CEM and JM tumor cells. Whether or not such effects can be employed for the treatment of such tumors must await future studies.