Hydroxychalcone derivatives belonging to polyphenol group, and distributed throughout the plant kingdom, can behave as anti-cancer agents due to their cytotoxic activities. Hydroxychalcone derivatives also have the potential as chemotherapeutic drugs for cancer. In this paper, we revealed that positions and numbers of hydroxyl groups in hydroxychalcones are involved in cytotoxicity against human monoblast U937 cells. Interestingly, 2-hydroxychalcone remarkably reduced viability of U937 cells as compared to chalcone, 2'-hydroxychalcone, 4-hydroxychalcone, and 4'-hydroxychalcone. In addition, 2', 4, 4'-trihydroxychalcone showed stronger cytotoxicity than 2'-hydroxychalcone, 2', 4-dihydroxychalcone, and 2', 4'-dihydroxychalcone. These results demonstrate that positions and numbers of hydroxyl groups in hydroxychalcones are involved in cytotoxicity against U937 cells. Moreover, we showed that all-trans retinoic acid-induced differentiation of U937 cells brought about an enhanced resistance against some cytotoxic hydroxychalcone derivatives. These data suggest that hydroxychalcone derivatives in combination with all-trans retinoic acid could serve as effective modifiers in therapy for leukemia.