CT without contrast of lumbosacral nerve roots was performed in 13 patients with peroneal atrophy and 28 control subjects. Two series of 5 mm serial sections parallel to the plane of the disk were examined at the L4-L5 and L5-S1 levels, and the transverse diameter of the S1 nerve roots measured at the lower part of the lateral recess. Results showed frank bilateral, grossly symmetrical hypertrophy of lumbosacral roots in 8 of the 13 patients. This hypertrophy involved all roots examined (L4, L5, S1), except in one case where only S1 roots were involved. Hypertrophy was often more marked on the distal part of the roots and on spinal nerves, contrasting with the sometimes normal or only slightly altered appearance of the nerve roots emerging from the dural sac. In these 8 cases, the diameter of the S1 nerve roots was 8 to 18 mm, in contrast to a mean of 3.5 +/- 1 mm in the 28 controls. CT scan images were normal in the remaining 5 patients. The presence of a CT image of nerve hypertrophy was in all cases associated with a marked fall in nerve conduction rate (median nerve motor conduction rate less than 25 msec-1), and a decrease in number of myelinated fibers with numerous onion bulbs. In contrast, the absence of CT nerve hypertrophy could not predict the results of electrophysiological and histological examinations.