Background: Breast cancer is reported to be the leading cause of cancer morbidity and mortality among Nigerian women. Deficiency of some trace elements such as Selenium has been implicated in the etiopathogenesis of breast disease particularly breast cancer. Aim: To determine the relationship of Selenium to the occurrence of breast diseases through the evaluation and comparison of its tissue levels in patients with cases of fibroadenoma and cancer of the breast and those in the control group. Method: Consecutive consenting patients who had histologically confirmed breast cancer and fibroadenoma, as well as, control subjects attending General surgical outpatients departments of Obafemi Awolowo University Teaching Hospital Complex, Ile-Ife, were recruited for the study. One gram of disease breast tissue and 5 mls of whole blood were taken for Selenium analysis in Cancer and Fibroadenoma group while only 5 mls whole blood was taken from control group. For mastectomy specimens, the tumor was bisected into half and the samples were taken from the core of the tissue so as to ensure that disease part was sampled while part of the excised fibroadenoma specimen was taken for Selenium analysis. The specimens were analyzed using graphite furnace atomic absorption spectrometer. Subjects’ demographics and clinical data were recorded on a standard proforma. Result: There were 167 female subjects in the 3 groups. There were 95 (57%) cases of fibroadenoma and 32 (19%) of breast cancer. There were also 40 (24%) subjects in the control group. The age range was 16 to 70 years, median of 32 years. The educational and socioeconomic statuses of the three groups were statistically insignificant. While breast cancer was most common on the left, fibroadenoma was most common on the right breast ((χ = 8.994; p=0.011). The median whole blood Selenium level of the control subjects was 375.5µg/L (range =160- 610µg/L. For the patients with fibroadenoma, it was 360µg/L with a range of 150 to 601µg/L and 374.5µg/L with a range of 158 to 530 for the breast cancer group. The differences observed in the three group was not statistically significantly different (p=0.367). The median tissue level of Selenium in patients with fibroadenoma was 0.0272mg/g with a range of 0.0124 to 0.0576mg/g and that of the cancer patients which was 0.0178mg/g with a range 0.0072 to 0.0436mg/g were statistically significantly different ( p=0.001). Factors affecting tissue Selenium level include age (p<0.001), overall stage of breast cancer (p<0.001), maximum length of breast mass (p=0.023), previous delivery (p=0.004), age at last confinement (p=0.007), parity (p<0.001), Estrogen receptor (ER) status (p<0.001) and Progesterone receptor (PR) status (p=0.021). . Conclusion: Lower tissue Selenium level in breast cancer when compared with fibroadenoma may have a role in the occurrence of breast cancer. Increasing tissue level of breast tissue Selenium may prevent the occurrence of breast cancer