Between 1976 and 1990, we treated 56 isthmic spondylolisthesis patients with an average age of 15 + 4 (range, 4 + 9 to 27 +/- 2) with in situ bilateral transverse process fusions utilizing autogenous iliac bone graft. None of the patients had a decompression or instrumentation placed. Radiographic fusion was determined from standing spot lateral radiographs and either Ferguson AP or standing AP views of the lumbosacral junction. The radiographic fusion patterns were categorized into the following four grades: (A) definitely solid (n = 28, 50%)--solid big trabeculated bilateral fusion masses; (B) possibly solid (n = 10, 18%)--unilateral large fusion mass with contralateral small fusion mass; (C) probably not solid (n = 6, 11%)--small, thin fusion masses bilaterally; (D) definitely not solid (n = 12, 21%)--graft resorption bilaterally or fusion mass with obvious bilateral pseudarthrosis. This fusion pattern is best determined by the Ferguson ("true AP") radiograph of the lumbosacral junction. Despite this surprisingly low fusion rate, overall clinical improvement was noted in > 80% of patients with preoperative symptoms of back pain, leg pain, or hamstring tightness. We recommend this critical assessment of fusion grades to determine fusion success in this patient population.