Two hundred and forty histological paraffin sections were obtained from the midportion of the spinal nerve roots of 20 lumbosacral plexus (from L4 to S3) bilaterally. All microscopic images were digitized using the NIH image software with a Nikon microscope and Sony videocamera. The total, fascicular as well as epineurial cross-sectional areas of the nerve roots in the lumbosacral plexus were determined. The total cross-sectional area of the lumbosacral trunk (LST) was the largest (28.56 +/- 12.28 mm2) followed by the S1 (21.97 +/- 11.22 mm2) and L5 (21.00 +/- 8.79 mm2) nerve roots. The total cross-sectional areas of the L4 (6.93 +/- 3.32 mm2), S2 (13.93 +/- 5.86 mm2) and S3 (6.03 +/- 3.74 mm2) were significantly lower. Statistical differences were found among all absolute values at different levels (p < 0.0001) with the exception of the levels between L4 and S3, L5 and S1, and LST and S1 (p > 0.05). The total areas occupied by the fascicles in L5 (7.78 +/- 3.26 mm2), LST (9.97 +/- 4.01 mm2) and S1 (8.55 +/- 3.27 mm2) nerve roots were greater than those in L4 (2.96 +/- 1.50 mm2), S2 (5.56 +/- 2.34 mm2) and S3 (2.28 +/- 1.14 mm2). However, the percentages of the total cross-sectional areas occupied by the fascicles in the L5 nerve root (38%) and LST (36.4%) were smaller compared to other nerve roots (44.9% at L4, 40.5% at S1, 40.8% at S2 and 41.6% at S3). The cross-sectional areas and percentages of the epineurium were greater in L5 (13.22 +/- 6.48 mm2, 62.0%), LST (18.58 +/- 9.31 mm2, 63.6%) and S1 (13.42 +/- 8.88 mm2, 59.5%) roots. The L5 (12.1 +/- 5.0), LST (27.5 +/- 11.4) and S1 (15.0 +/- 7.3) roots contained more fascicles than L4 (6.3 +/- 3.3), S2 (9.5 +/- 4.1) and S3 (7.1 +/- 2.7). Statistical differences were found among all absolute values at different levels (p < 0.0001) with the exception of the levels between L4 and S3, L5 and S1 and LST and S1 (p > 0.05). No statistical differences were found for percentages among different levels (p > 0.05) with the exception of the levels between L4 and L5, L4 and LST, LST and S1, LST and S2, and LST and S3 (p < or = 0.05). The histological structure of nerve roots of the lumbosacral plexus is identical to that of the peripheral nerve. The midportions of L5 and S1 roots and the LST have a relatively higher epineurial content. These nerve roots also have a greater number of the fasicles, but the total cross-sectional area occupied by the fascicles is less than in L4, S2 and S3 nerve roots.