Current data indicate that there is a significant risk of endometrial cancer (EC) in women with polycystic ovarian syndrome (PCOS), although further research needed to clarify the exact molecular mechanisms. Endometrial hyperplasia is a premalignant condition that usually heralds EC and it shares identical risk factors with EC. Metabolic syndrome with a triad of obesity, hyperinsulinaemia and diabetes, which is commonly observed in PCOS appears to be a key mechanism in EC pathogenesis. Measures to improve insulin resistance could therefore play a role in reducing the risk of EC in women with PCOS. Metformin is an insulin sensitising agent which is safe, widely available and currently licensed for type-2 diabetes. It has been clearly shown in both animal and human studies that metformin is of value in reversing endometrial hyperplasia. Metformin may therefore prevent EC in PCOS. This article reviews the use of metformin in reducing EC risk in PCOS and makes a case for future research on this topic.