We present the case of a 44-year-old nulliparous woman who experienced irregular menstrual cycles for about 10 years and developed both pituitary prolactinoma and endometrioid endometrial carcinoma. In premenopausal women, hyperprolactinemia causes hypogonadism by inhibiting secretion of gonadotropin-releasing hormone and thus suppressing luteinizing hormone levels, which can cause menstrual disorders ranging from amenorrhea, oligomenorrhea and chronic anovulatory cycle to short luteal phase of the menstrual cycle. A chronic anovulatory menstrual cycle is the most common cause of long-term exposure of the endometrium to endogenous estrogen without adequate opposition from progestins, which can lead to endometrioid endometrial carcinoma. In this case, pituitary prolactinoma may have caused the chronic anovulatory cycle and indirectly led to the endometrioid endometrial carcinoma. In patients for whom the cause of irregular menstruation and chronic anovulatory cycle is suspected to be hyperprolactinemia, explorations of both the hypophysis and endometrium are essential.