Soy isoflavones are compounds found in soybean and soybean products. They have been reported to possess numerous physiological properties, such as antitumor, anti-menopausal (female) osteoporosis and anti-aging. They have also been reported to improve learning and memory skills in menopausal women and aid in the prevention and treatment of heart disease, diabetes and Kawasaki disease (KD). In this review, the effects of soy isoflavones on various diseases were analyzed. Based on the analysis, it was hypothesized that the function of soybean isoflavones in the prevention and treatment of various diseases results from their phytoestrogen and antioxidant properties. However, due to their phytoestrogen properties, it is recommended that the risks of soy isoflavone intake as food and/or medical treatment be further evaluated.