Heavy menstrual bleeding is a type of abnormal uterine bleeding. Abnormal uterine bleeding includes the poorly characterized "not otherwise classified" category. Here, we report three abnormal uterine bleeding-not otherwise classified cases with uniform thickening of the junctional zone endometrium. Case 1: A 33-year-old nullipara with heavy menstrual bleeding presented with severe anemia (hemoglobin: 4.7 g/dL) and an 8.4-mm junctional zone endometrium on magnetic resonance imaging. Her condition improved with iron and low-dose estradiol-progestins. Case 2: A 36-year-old nulligravida had heavy menstrual bleeding, anemia (hemoglobin: 9.5 g/dL), and a 9.4-mm junctional zone endometrium; her anemia improved with iron supplementation. Case 3: A 39-year-old multipara had heavy menstrual bleeding, anemia (hemoglobin: 9.6 g/dL), and a 12.3-mm junctional zone endometrium, and was managed with a levonorgestrel-releasing intrauterine system. Pelvic examination, transvaginal sonography, and uterine size on magnetic resonance imaging were normal in all cases. In those without uterine abnormalities, uniform thickening of the junctional zone endometrium (⩾8 mm) may trigger heavy menstrual bleeding; hence, magnetic resonance imaging may be warranted in abnormal uterine bleeding-not otherwise classified cases.