Although the role of carotenoids in the prevention of breast cancer has been studied more extensively, human studies with lycopene and beta-carotene on cancer are now being undertaken. The majority of studies on the effects on the cell cycle were carried out after 48 h of carotenoid treatment, which can lead to underestimation of the effect of these substances. The aim of the present study was to determine the effect of lycopene and beta-carotene on cell cycle and cell viability in three different human breast cancer cell lines at different time points.