Hysteroscopy is the treatment of choice of benign intrauterine conditions, whenever feasible. During hysteroscopic procedures performed with the intent of resecting myomas and polyps, there is a potential risk of an unexpected diagnosis of an occult malignant lesion. Objective: To estimate the incidence of occult uterine malignancy in women undergoing hysteroscopic electroresection due to a diagnosis of benign uterine lesions: endometrial polyps and submucosal myomas, by ultrasound. Material and methods: The electronic database of patients hospitalized between January 2010 and December 2016 in the Department of Gynecology and Oncology of the Jagiellonian University was searched for women who had undergone hysteroscopic surgery due to presumed endometrial polyps and submucosal myomas. Medical records of 1,006 eligible women were analyzed. Results: Ten cases (10/1,006; 1%) of occult endometrial cancer, 2 cases (2/1,006; 0,2%) of low-grade endometrial stromal sarcoma and 3 cases (3/1,006; 0,3%) of atypical endometrial hyperplasia were found postoperatively. One case of ovarian cancer coexisting with endometrial cancer was diagnosed. Of the factors analyzed, only age, menopausal status and abnormal uterine bleeding turned out to be significant endometrial cancer risk factors. Too few sarcoma cases did not allow similar estimates. We did not observe worsening of the prognosis, and all 15 patients who underwent hysteroscopic resection of the unforeseen malignancy or precancerous lesion are still alive within 2–8 years of follow-up. Conclusions: The incidence of unlooked-for malignancy in patients with benign intracavitary lesions, i.e. endometrial polyps and submucosal myomas, is relatively high. No negative effects of hysteroscopic resection on potential further treatment and prognosis have been proven so far. High risk of malignancy in women with presumed benign intracavitary lesions may indicate that not all these women are appropriate candidates for laparoscopic procedures with power morcellation, particularly when no previous histopathological verification is ordered.