Anthocyanins are pigments in plants. Anthocyanins and their aglycons, anthocyanidins, have various biological effects. In the present study, we examined the effects of cyanidin and cyanidin-3-glucoside on intracellular reactive oxygen species (ROS) levels and on proliferative effects, using four cancer cell lines and one normal cell line, and we found that cyanidin (100 μmol/L) has significant effects when compared with cyanidin-3-glucoside. Cyanidin reduced intracellular ROS levels in MCF-7, HuH-7, HepG2, Caco-2 and HUVEC cell lines, but cyanidin-3-glucoside did not. Cyanidin inhibited the proliferation of MCF-7, HuH-7, HepG2 and Caco-2 cells, but cyanidin-3-glucoside did not. Cyanidin and cyanidin-3-glucoside had no effect on the proliferation of HUVEC. We also tested cyanidin-3-rutinoside, peonidin and peonidin-3-glucoside, but the presence of a sugar in their structure hampered the biological effects of aglycons. These results suggest that a sugar in the anthocyanin structure affects biological activity.