OBJECT: The study aims to analyze nerve fiber types in the sacral nerve roots as a prerequisite for stimulation. METHODS: One-micrometer cross-sections of human ventral and dorsal S1-5 roots were stained with osmium and toluidine blue. The total fiber diameter and myelin sheath were measured in 282,420 nerve fibers. RESULTS: The analysis revealed the following 3 main nerve fiber types: Aalpha fibers (diameter 6-14 microm), Agamma fibers (diameter 2-4 microm), and B fibers (diameter < 2 microm). The B fibers were absent in S-1, present in some S-2 fascicles, and abundant from S-3 to S-5. The Aalpha fibers dominated the S-1 roots and most fascicles of S-2 roots. In the S3-5 roots, only a few Aalpha fibers were present. The relative occurrence of Agamma fibers increased from S-1 to S-5. In dorsal roots, Agamma fibers represented approximately 70% of all nerve fibers in every root and fascicle. CONCLUSIONS: The B fibers represented efferent parasympathetic fibers. These fibers were concentrated in certain areas of the nerve roots, not randomly distributed. The Aalpha fibers innervate lower-extremity muscles and sphincters. The inverse correlation of Aalpha and Agamma fibers in the ventral roots from S-1 to S-5 is surprising. In dorsal roots, Agamma fibers may conduct pain, touch, and temperature signals. Highly selective fiber stimulation specific for type, location, and direction may improve sacral nerve stimulation for a spastic bladder in paraplegic individuals.