Rhododendron luteum belongs to the genus Rhododendron, members of which are frequently used in folk medicine. Various studies have investigated the cytotoxic effect of different Rhododendron species, but there have been limited studies of the cytotoxic effect of R. luteum. The aim of this study was to investigate the antioxidant properties and cytotoxic effect of R. luteum leaf extract on the human cancer cells. The total phenolic content (TPC), total flavonoid content (TFC), and reducing power of the extract were evaluated using spectrophotometric procedures. The cytotoxic effect of the extract on five cancers (human breast, colon, lung, prostate, and liver carcinoma) and human fibroblast cells was determined using the MTT assay. TPC and reducing power values of extract were found 173.2 +/- 3.97 mg gallic acid equivalents, and 629.5 +/- 3.49 mg trolox equivalents per g sample, respectively. R. luteum leaf extract exhibited selective cytotoxicity especially against colon (1.9 fold) and liver (2.2 fold) cancer cells compared to normal fibroblast cells. This is the first study to reveal the cytotoxic effect of R. luteum leaf extract. Further studies are now needed to identify the cytotoxic molecules in the extract and their mechanisms.