Purpose This study was designed to (1) evaluate the clinical outcomes after arthroscopic subspinal decompression in patients with hip impingement symptoms and low AIIS, and to (2) assess the presence of low anterior inferior iliac spine on the pre-operative radiographs of patients with established subspinal impingement diagnosed intra-operatively. Methods Retrospective analysis of patients who underwent arthroscopic subspinal decompression has been performed. The indications for surgery were femoroacetabular impingement (FAI), or subspinal impingement. Pre-operative radiographs were assessed for anterior inferior iliac spine type. Intra-operative diagnosis of low anterior inferior iliac spine was based on the level of anterior inferior iliac spine extension relative to the acetabulum and the presence of reciprocal labral and chondral lesions. In patients where low anterior inferior iliac spine was not diagnosed on pre-operative radiographs, the pre-operative radiographs were re-read retrospectively to assess missed signs of low anterior inferior iliac spine. Results Thirty-four patients underwent arthroscopic subspinal decompression between 2012 and 2015. The patients were followed for a median of 25 months (13–37 months). Intra-operatively, grade 2 anterior inferior iliac spine was found in 27 patients and grade 3 anterior inferior iliac spine was found in 7 patients. MHHS, HOS, and HOSS scores increased from median (range) pre-operative scores of 55 (11–90), 48 (20–91) and 20 (0–80) to 95 (27–100), 94 (30–100) and 91 (5–100), respectively ( p  < 0.0001, p  = 0.001, p  < 0.0001, respectively). Pre-operative diagnosis of low AIIS was made in 6/34 patients via AP radiographs. On retrospective analysis of pre-operative radiographs, signs of low AIIS were still not observed in 21/34 (61.8%) patients. Conclusions Arthroscopic subspinal decompression of low AIIS yielded significantly improved outcome measures and high patient satisfaction at a minimum of 13 months follow-up. Low AIIS is often under-diagnosed on AP pelvis and lateral frog radiographs and if left untreated, may result in unresolved symptoms and failed procedure. Level of evidence IV.