Low back pain persisting or appearing after a technically successful lumbar fusion challenges clinicians. In this context, the sacroiliac joint could be a possible source of pain, but the frequency of its responsibility is not really known. We used sacroiliac anesthetic blocks, the gold standard for diagnosis, to determine this frequency. Our second goal was to search predictive factors for a positive block. Our prospective series consisted of 40 patients with persistent low back pain after a technically successful fusion who received a sacroiliac anesthetic block under fluoroscopic control. The diagnostic criterion was a relief of more than 75% of the pain on a visual analog scale. We found a 35% rate of positive blocks. The only criterion that characterized these patients was a postoperative pain different from the preoperative pain in its distribution ( p =0.017). A free interval of more than 3 months between surgery and appearance of the pain had an indicative value ( p =0.17). An increased uptake in the sacroiliac on bone scintigraphy or a past history of posterior iliac bone-graft harvesting had no significant value ( p =0.74 and p =1.0, respectively). The sacroiliac joint is a possible source of pain after lumbar fusion. The anesthetic block under fluoroscopic control remains the gold standard.