The purpose of this study is to clarify sensory innervation in the ventral and dorsal sides of the sacroiliac joint. Fluoro-gold, a neural tracer, was injected into the left sacroiliac joint of adult rats from the dorsal side after denervation of the dorsal side, and the bilateral dorsal root ganglia (DRGs) from T13 to S4 were examined by fluorescence microscopy five days after injection. In another rat group, the DRGs were examined using the same methods after injection of fluoro-gold from the ventral side. In the case of dorsal denervation (ventral nerve supply), labeled neurons were mainly located in the ipsilateral DRGs from L1 to S2. On the other hand, in the case of ventral denervation (dorsal nerve supply), labeled neurons were noted in the ipsilateral DRGs from L4 to S2. The sacroiliac joint in rats is innervated differently on the ventral and dorsal sides: the sensory nerve fibers to the dorsal side of the sacroiliac joint were derived from the DRGs of lower lumbar and sacral levels (from L4 to S2); and those to the ventral side from the DRGs of upper lumbar, lower lumbar, and sacral levels (from L1 to S2). © 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.