Amsonia orientalis is an ornamental plant with anti-cancer properties. To characterize the anti-cancer potential of the species, methanolic extracts were prepared from leaf and stem tissues of field-grown individuals. Also, a mixture of stems, leaves and roots of in vitro grown plantlets (SLR) were extracted with methanol and used. Effects of these extracts on the cancer cell lines MCF-7 (breast) and DU145 (prostate) were investigated. After incubation for 48 h, the leaf extract gave the lowest IC50 values (197.8 mu M for MCF-7 and 300.9 mu M for DU145). The SLR extract also gave the similar results, but with higher IC50 values. The stem extract had the weakest effect on cancer-cell-specific cytotoxicity. The extracts did not have a significant effect on DU145, compared to fibroblasts. The leaf extract increased the number of active-Caspase-3 positive cells in MCF-7 culture slightly, but the main effect of all extracts was on the cell proliferation rather than apoptosis. Real-time cellular analysis of MCF-7 demonstrated a 1.8-fold increase of doubling time for the leaf extract. The expression of proliferation-related gene MCM2 was found to be decreased in MCF-7 cells treated with the leaf extract, while its expression increased in fibroblasts after the same treatment. In conclusion, the extracts affected the MCF-7 cells at moderate concentrations. This study suggests that the leaf methanolic extracts might be a good source of compounds with cytostatical properties.