STUDY OBJECTIVE To determine the prevalence of adenomyosis and deep adenomyosis after NovaSure (Hologic Inc., Newark, DE) endometrial ablation in hysterectomy specimens after NovaSure endometrial ablation failure. DESIGN Prospective observational study (Canadian Task Force classification II-2). SETTING The TweeSteden Hospital, a teaching hospital in the south of the Netherlands. PATIENTS All women who underwent hysterectomy for menorrhagia and/or dysmenorrhea after failure of NovaSure endometrial ablation between November 2007 and January 2011. INTERVENTIONS All patients who underwent hysterectomy between January 2005 and April 2009 for the same indication but did not undergo prior endometrial ablation from choice or the lack of availability of the endometrial ablation procedure. MEASUREMENTS AND MAIN RESULTS Of 213 patients who underwent NovaSure therapy, 22 (10.3%) underwent a hysterectomy because NovaSure failed. Of these, 10 (45.5%) exhibited adenomyosis in their hysterectomy specimens. The control group patients had a similar adenomyosis prevalence (74/173, 42.8%). However, the NovaSure failure group had a significantly higher prevalence of deep adenomyosis (>2.5 mm endometrial penetration) (9/22, 40.9%) than the control group (37/173, 21.4%, p < .05). CONCLUSION Deep adenomyosis after failed NovaSure endometrial ablation was present in a significant number of patients. It is not clear whether adenomyosis is induced by endometrial ablation or whether it causes endometrial ablation failure.