Cerebrospinal fluid (CSF) bathes and protects our brain and spinal cord. Examining this fluid can be important in determining if an infection or tumor has spread to this part of the body. The fluid is obtained by a lumbar puncture or 'spinal tap'. The patient is placed on his or her side with the knees tucked to the chest in a fetal position. An area in the middle of the low back is numbed with local anesthetic. A needle is then inserted and fluid removed. The procedure generally takes less than a minute or two. In experienced hands, this procedure is minimally painful.