BACKGROUND: Thermal balloon endometrial ablation is a relatively safe nonsurgical treatment for menorrhagia. Hematometra follows this procedure in <3 % of patients, but risk factors for this complication are unclear. CASE: A woman with a history of cervical incompetence during pregnancy later developed cervical occlusion and hematometra after thermal balloon endometrial ablation. Cervical occlusion did not recur after cervical dilatation and temporary placement of a catheter as a stent. CONCLUSION: The normal resistance of the internal cervical os may be an important factor in avoiding thermal damage to the cervix during thermal balloon endometrial ablation. This case suggests that a history of cervical incompetence may be a clinical indicator of decreased cervical resistance.