Selenium (Se) is known to regulate carcinogenesis and immunity at nutritional and supranutritional levels. Because the immune system provides one of the main body defenses against cancer, we asked whether T cell immunity can modulate selenium chemoprevention. Homozygous NU/J nude mice were fed selenium-deficient, torula yeast basal diet alone or supplemented with 0.15 or 1 mg Se/kg for 8 months in Experiment 1 and for 11 weeks in Experiment 2. Mice were inoculated with PC-3 prostate cancer cells followed by a 7-week tumor development. Peripheral T cells were analyzed at baseline, day 9, 19, 34 and 47 by flow cytometry. Tumor development in adult nude mice (Experiment 1) was suppressed whereas in young nude mice (Experiment 2) was promoted by feeding a high selenium diet. Dietary selenium deficiency does not affect tumor weight. After xenograft, dietary selenium status does not affect levels of CD4 and CD8 T cells in adult nude mice in Experiment 1, while high selenium resulted in significant decrease in CD4 T cells in young nude mice in Experiment 2. Taken together, there is an opposing role of excessive selenium on tumor xenograft development in adult and young nude mice carrying differential T cell profiles.