The present study focused on the elucidation of the putative anticancer potential of quercetin. The anticancer activity of quercetin at 10, 20, 40, 80 and 120 µM was assessed in vitro by MMT assay in 9 tumor cell lines (colon carcinoma CT‑26 cells, prostate adenocarcinoma LNCaP cells, human prostate PC3 cells, pheocromocytoma PC12 cells, estrogen receptor‑positive breast cancer MCF‑7 cells, acute lymphoblastic leukemia MOLT‑4 T‑cells, human myeloma U266B1 cells, human lymphoid Raji cells and ovarian cancer CHO cells). Quercetin was found to induce the apoptosis of all the tested cancer cell lines at the utilized concentrations. Moreover, quercetin significantly induced the apoptosis of the CT‑26, LNCaP, MOLT‑4 and Raji cell lines, as compared to control group (P<0.001), as demonstrated by Annexin V/PI staining. In in vivo experiments, mice bearing MCF‑7 and CT‑26 tumors exhibited a significant reduction in tumor volume in the quercetin‑treated group as compared to the control group (P<0.001). Taken together, quercetin, a naturally occurring compound, exhibits anticancer properties both in vivo and in vitro.