Many women would prefer fewer bleeding episodes while taking oral contraceptives. For this reason and with the intention of reducing menstruation-associated symptoms, an extended-cycle contraceptive is considered in the present paper. However, it remains unknown whether this long-term treatment is associated with a different breast cancer risk from that of the usual treatment. Therefore, in the present in vitro work we intend to compare the effect of these different treatment regimens on breast cancer risk. MCF-7 cells (human estrogen- and progesterone-receptor-positive metastatic breast cancer cells) and HCC1500 cells (human estrogen- and progesterone-receptor-positive primary breast cancer cells) were incubated with physiological concentrations of ethinylestradiol (EE). Usual and extended cycles were mimicked by incubation periods of 18 hours with EE followed by 6 hours without EE and 24 hours with EE for 3 days, respectively. In both cell lines, EE elicited a significant increase in the proliferation rate. No significant difference was found between the two incubation periods. Our results indicate that continuously administered ethinylestradiol may not increase breast cancer risk in comparison to intermittent application. However, clinical studies are necessary to prove these in vitro results.