Tarlov's cyst is a usually congenital (i.e. you were born with it) type of disorder. This is a spinal fluid filled cyst that sits long the remnants of the spinal cord as they decend through the sacrum (usually). Often, they are found when an MRI is ordered for back issues but most often they do not cause pain and thus they are incidental findings. In the rare occassions that they do cause pain the problems are usually due to compression of the lower sacral nerves and this results in groin regional type pain or numbness. Patients also may develop bladder or bowel issues since the lower sacral nerves innervate the rectum and lower urinary system. Before blaming the Tarlov's Cyst other causes should be ruled out since the surgery to correct the cyst is not minor in most cases. Often the problem is due to stenosis or cord compression higher up.