Xanthohumol (XN) and 8-prenylnaringenin (8PN) are hop (Humulus lupulus L.) polyphenols studied for their chemopreventive effects on certain cancer types. The breast cancer line MCF-7 was treated with doses ranging from 0.001 to 20 µM of XN or 8PN in order to assess the effects on cell viability and oxidative stress. Hoechst 33342 was used to measure cell viability and reactive oxygen species (ROS) production was determined by 2′,7′-dichlorofluorescein diacetate. Catalase, superoxide dismutase, and glutathione reductase enzymatic activities were determined and protein expression of sirtuin1, sirtuin3, and oxidative phosphorylation system (OXPHOS) were done by Western blot. Treatments XN 0.01, 8PN 0.01, and 8PN 1 µM led to a decrease in ROS production along with an increase of OXPHOS and sirtuin expression; in contrast, XN 5 µM gave rise to an increase of ROS production accompanied by a decrease in OXPHOS and sirtuin expression. These results suggest that XN in low dose (0.01 µM) and 8PN at all assayed doses (0.001–20 µM) presumably improve mitochondrial function, whereas a high dose of XN (5 µM) worsens the functionality of this organelle. J. Cell. Biochem. 114: 2785–2794, 2013. © 2013 Wiley Periodicals, Inc.