Up to 14 percent of women experience irregular or excessively heavy menstrual bleeding. This abnormal uterine bleeding generally can be divided into anovulatory and ovulatory patterns. Chronic anovulation can lead to irregular bleeding, prolonged unopposed estrogen stimulation of the endometrium, and increased risk of endometrial cancer. Causes include polycystic ovary syndrome, uncontrolled diabetes mellitus, thyroid dysfunction, hyperprolactinemia, and use of antipsychotics or antiepileptics. Women 35 years or older with recurrent anovulation, women younger than 35 years with risk factors for endometrial cancer, and women with excessive bleeding unresponsive to medical therapy should undergo endometrial biopsy. Treatment with combination oral contraceptives or progestins may regulate menstrual cycles. Histologic findings of hyperplasia without atypia may be treated with cyclic or continuous progestin. Women who have hyperplasia with atypia or adenocarcinoma should be referred to a gynecologist or gynecologic oncologist, respectively. Ovulatory abnormal uterine bleeding, or menorrhagia, may be caused by thyroid dysfunction, coagulation defects (most commonly von Willebrand disease), endometrial polyps, and submucosal fibroids. Transvaginal ultrasonography or saline infusion sonohysterography may be used to evaluate menorrhagia. The levonorgestrel-releasing intrauterine system is an effective treatment for menorrhagia. Oral progesterone for 21 days per month and nonsteroidal anti-inflammatory drugs are also effective. Tranexamic acid is approved by the U.S. Food and Drug Administration for the treatment of ovulatory bleeding, but is expensive. When clear structural causes are identified or medical management is ineffective, polypectomy, fibroidectomy, uterine artery embolization, and endometrial ablation may be considered. Hysterectomy is the most definitive treatment.