Objectives: 1) Examining the effectiveness of an outpatient aspiration biopsy from the uterine cavity in the context of satisfactory sampling. 2) Assessing risk factors of obtaining a non-diagnostic sample. 3) Search for ultrasound image patterns according to the International Endometrial Tumor Analysis that may indicate potential ineffectiveness of the biopsy. 4) Assessment of effectiveness of an outpatient biopsy in detection of endometrial cancer. Methods: Observational prospective study on a non-selected population of women suspected of endometrial pathologies. Results: 116 endometrial aspiration biopsies have been analyzed. Diagnostic histopathological samples were obtained in 100 cases (86.2%). There was no statistical significance in obtaining a non-diagnostic result of a biopsy in terms of the menopausal status, presence of uterine fibroids, occurrence of abnormal uterine bleeding, age, endometrial thickness and the number of deliveries. A large variety of ultrasound images were obtained according to the International Endometrial Tumor Analysis parameters. There were no clearly dominant patterns of endometrial ultrasound parameters in patients with non-diagnostic biopsy results. 100% of endometrial cancer cases were reported in postmenopausal patients with the history of abnormal uterine bleeding. The mean thickness of the endometrium was statistically the largest in this group: 16.2 mm (4–22.3 mm) (p = 0.025). Conclusions: An outpatient endometrial aspiration biopsy is an effective, low-impact procedure in case of indications for histopathological assessment of the endometrium.