Irregular bleeding is a common complaint of adolescents and is responsible for approximately 50% of gynecologic visits in that age group. Most abnormal bleeding in adolescents is caused by immaturity of the hypothalamic-pituitary-ovarian axis resulting in anovulation. Approximately 20% of adolescents have an underlying disorder requiring targeted diagnostic testing. A thorough history and physical examination and laboratory findings will often reveal the source of abnormal bleeding. Appropriate treatment is then directed to the underlying cause.