In thirty-two patients who demonstrated a complete or almost complete block on a lumbar myelogram, computerized tomography of the non-visualized vertebral levels caudad to the block was performed prior to surgical intervention. The purpose of this study was to evaluate the clinical value of computerized tomography in detecting a lesion that is caudad to the level of a myelographic block. For twenty-three patients the cause of the myelographic block was stenosis of the spine; for five patients, a combination of stenosis of the spine and herniation of a disc; for one patient, herniation of a disc between the fourth and fifth lumbar vertebrae alone; for two patients, arachnoiditis; and for one patient, kyphosis secondary to fracture. A total of fifty vertebral levels that could not be visualized because of the block were evaluated. Thirty (60 per cent) of the non-visualized vertebral levels, in nineteen (59 per cent) of the thirty-two patients, demonstrated stenosis of the spine or a herniated disc that was confirmed at the time of surgical treatment. The value of computerized tomography for the evaluation of the vertebral levels caudad to the level of a complete or almost complete block on a lumbar myelogram was threefold. First, it provided visualization of the vertebral levels that could not be evaluated by the myelography. Second, the findings on computerized tomography provided information that was essential for preoperative planning and it removed the so-called exploratory element from the operative procedure.(ABSTRACT TRUNCATED AT 250 WORDS)