Optimal in vitro conditions regarding quercetin solubility and stability were defined. Using these conditions, the effect of quercetin on proliferation of the colon carcinoma cell lines HCT-116 and HT29 and the mammary adenocarcinoma cell line MCF-7 was investigated. For the colon carcinoma cell lines, at relatively high concentrations, a significant decrease in cell proliferation was observed, providing a basis for claims on the anti-carcinogenic activity of quercetin. However, at lower concentrations, a subtle but significant stimulation of cell proliferation was observed for all cell lines tested. These results point at a dualistic influence of quercetin on cell proliferation that may affect present views on its supposed beneficial anti-proliferative effect.