The sacroiliac joint (SIJ) can cause pain after lumbosacral fusion. Diagnosis requires >75% relief after local anesthetic SIJ injection. This study is a retrospective review of patients with low back pain after lumbosacral fusion who had SIJ injections. Percentage and duration of pain relief were noted. Results are as follows: there were 34 patients; 8 fused at L5–S1, 14 fused at L4–S1, and 12 had multilevel fusions. Twenty had >75% relief within 45 minutes, and 11 had prolonged relief. Six had relief >20% but <75%, and one had prolonged relief. Eight never improved. Eight had posterior iliac crest bone harvested, and there was no correlation between donor side and pain side. In 34 patients with low back pain after lumbosacral fusion, SIJ was the cause of pain in 32% and possibly the cause in 29%. This is the first detailed description of this problem.