Background:Spondylolysis and spondylolisthesis can be associated with significant low back pain Respecially in physically active adolescents. Non-operative management is usually successful in improving symptoms, but surgical intervention is  occasionally required. The aim of this study was to determine the effect of in situ posterolateral fusion in the treatment of refractory cases with spondylolysis.Methods:In this prospective before and after study, we described our experience  in13 patients managed by in situ fusion after failing multimodality non-operative  treatment. All surgical procedures were performed by the senior author and by a similar technique.The spondylolytic vertebra and the one below were fused, in situ. Finally, clinical outcome and recovery rates of clinical symptoms were evaluated by  Henderson’s functional capacity and Oswestry Disability Index version 2.1, respectively.  Results: The mean duration of non-operative management was 36 (12-72)  months. There were 8 males and 5 females. Average pre- and postoperative Oswestry  Disability Indices were 28.4%±13.7% and 4.9±7.8 respectively (P=0.001, significant). All patients had follow-up contact on an average of 42.3 months (range 30 - 62  months). Based on Henderson’s clinical outcome functional capacity at the final follow- up stage clinical outcomes were excellent in 10, good in 2 and poor in 1 patient.The case with poor result had a pseudoarthrosis and was re-oprated. Finally he had an  excellent outcome.Conclusion: We accept that the number of our cases is not high significantly but it can be claimed that in situ fusion is a safe and effective modality to treat symptomatic  patients with spondylolysis and low-grade spondylolisthesis. A study with much  more cases is strongly recommended.