Endocervicosis, a mucinous analogue of endometriosis, is a rare, non-neoplastic mullerian lesion that may involve the urinary bladder. This lesion has only recently been described in the pathologic literature and, to the best of our knowledge, it has not been previously reported in the urologic literature. It is a benign lesion that clinically may be confused with a malignant tumor. Histologically, it must be differentiated from an adenocarcinoma. We report one recent case of urinary bladder endocervicosis in a 34-year old female who presented with urinary urgency and frequency secondary to a 3.5 cm mass in the posterior wall of the bladder. This was treated by partial cystectomy with complete resolution of her symptoms. The literature on endocervicosis and related mullerian lesions of the urinary bladder is reviewed.