Out of all the risk factors of endometrial carcinoma, the 'Oestrone Theory' is especially fascinating. This theory assumes that prolonged uninterrupted stimulation of the endometrial cell by oestrone, without the competition of oestradiol and oestriol, brings about neoplasia. There is also some evidence of a protective effect of androgens and progesterone. In order to verify this theory in vivo, oestrone (E1), oestradiol (E2), testosterone (T) and progestogen levels were examined from the serum of 36 post-menopausal women suffering from endometrial carcinoma, and compared with a healthy group of women. Oestrone levels were found to be significantly higher in patients with endometrial carcinoma. A correlation was also found between oestrone levels and previous infertility. With the intention of tracking the origin of oestrone in those suffering from endometrial cancer, hormone levels were checked before and after total hysterectomy and oophorectomy. The high oestrone values dropped to normal after the operation, except in those who had previously suffered from infertility. In those patients, high oestrone values were found a year after the operation. No significant differences of testosterone or progesterone were found between the cancer patients and the healthy women. The operation did not have any influence on these values.