Operative hysteroscopy is a relatively new technique that has significantly improved the diagnosis and therapy of abnormal uterine bleeding. At first, the success of operative hysteroscopy in controlling this bleeding seemed extremely high but, with long-term follow-up, a significant failure rate became evident requiring a repeat hysteroscopic procedure or a hysterectomy. Deep adenomyosis is a major cause of these failures. This paper describes three operative ablation techniques and relates many of their failures to deep adenomyosis. The definition and pathophysiology of adenomyosis are also explored. The possibility of delaying the diagnosis of endometrial cancer under an ablation scar is discussed. Ultimately the depth of adenomyosis seems to correlate with the outcome of endometrial ablation or resection. Patients without or with only minimal endometrial penetration of <2.5 mm (superficial adenomyosis) have good results from the ablation. Patients with deep endometrial penetration of >2.5 mm (deep adenomyosis) usually have persistent problems and should be offered hysterectomy over repeat ablation. Magnetic resonance imaging or ultrasound may be an appropriate pre-operative screening tool to determine the depth of ademomyosis.