OBJECTIVE The genus Rhododendron is distributed entirely in the world with the exception of South and Central America and Africa, growing in a large diversity of climatic conditions. This genus is a rich source of phenolic compounds, especially flavonoids, essential oils, chromones, terpenoids, and steroids. It has many biological properties such as antioxidant, anti-inflammatory, antiviral, antibacterial, anticancer, antidiabetic, immunomodulatory, cardioprotective and hepatoprotective among others due to their polyphenolic constituents. The objective of the current study was to evaluate the antioxidant properties and cytotoxic activity of dimethyl sulfoxide extract of flowers of Rhododendron luteum (DEFR) for the first time. METHODS The total polyphenolic contents (TPC), total flavonoid contents (TFC) and ferric reducing antioxidant power (FRAP) of the extract were evaluated using spectrophotometric procedures. The cytotoxic activity of the extract on three cancers (human breast, colon and liver carcinoma) and human foreskin fibroblast cells was determined using the MTT assay. RESULTS TPC and FRAP values were found 54.2±0.38 mg gallic acid equivalents and 164.2±1.77 mg trolox equivalents per to g sample, respectively. R.luteum extract exhibited selective cytotoxicity against colon and liver cancer cells compared to normal fibroblast cells, while this selective cytotoxicity was not observed in breast cancer cells. CONCLUSIONS Our results demonstrate that the Rhododendron luteum may be a great source of antioxidant and antitumor natural agents due to their capability of decreasing cancer cells proliferation.