Thirty-nine postero-lateral intertransverse spine fusions performed for isthmic spondylolisthesis of L5 in 38 patients and L4 in one patient were reviewed clinically and radiologically at a mean of 7.5 years following operation. In thirty-three patients, fusion was combined with excision of the mobile posterior vertebral arch. In thirty-seven patients, the graft was applied in situ and in two patients after pre-operative reduction with halo-femoral traction. There were 87 per cent of excellent and good clinical results and a fusion rate of 94.6 per cent. This simple and reliable technique gives results comparable with other more complex methods of spine fusion. It is, however, unsatisfactory in cases with a displacement greater than 60 per cent, requiring pre-operative reduction, Removal of the mobile posterior arch (Gill) did not significantly improve the results.