Background: The role of hip arthroscopy in dysplasia is controversial. Purpose: Determine the 7-year joint preservation rate following hip arthroscopy in hip dysplasia and identify anatomical and intra-operative features that predict success of hip preservation with arthroscopic surgery allowing formulation of an evidence-based classification. Study Design: Cohort Study; Level of evidence: 3 Methods: Between 2008 and 2013, 111 hips with dysplastic features [acetabular index (AI) > 10° and/or centre-edge angle (CEA) <25°] having undergone an arthroscopy were identified. Clinical, radiological and operative findings and type of procedure performed were reviewed. Radiographic evaluations of the operated hip [acetabular index (AI), centre-edge angle (CEA), extrusion index] were performed. Outcome measures included whether the hip was preserved at follow-up, pre- and post-operative NAHS and HOOS scores. We calculated AI and CEA factored (AIf and CEAf respectively) by a measure of articular wear as follows: AIf = AI x (number of UCL wear zones +1) CEAf = CEA / (number of UCL zones + 1) A contour plot of the resulting probability value of failure for every combination of AIf and CEAf allowed for the determination of the zones with the lowest and highest incidence of failure to preserve the hip respectively. Results: The mean AI and CEA were 7.8° and 18.0°, respectively. At a mean follow-up of 4.4 years, 33 hips had failed requiring a hip arthroplasty. The 7- year joint survival was 68%. The mean improvement in NAHS and HOOS were 7.8 and 23 points respectively. The zone with the greatest chance of joint preservation (odds ratio: 10, p<0.001) was AIf: 0 – 15 and CEAf: 15 – 25 (Green Zone); on the contrary the zone with the greatest chance of failure (odds ratio: 10, p<0.001) was AIf: 20 – 100 and CEAf : 0 – 10 (Red Zone). Conclusion: Overall, the 7- year hip survival in hip dysplasia appears inferior compared to reports of Femoro-Acetabular Impingement cases. Hip arthroscopy is associated with excellent chance of hip preservation in mild (Green Zone) dysplasia (AI< 15° & CEA: 15 – 25°) and no (or little) articular wear. Hip arthroscopy should not be performed in cases with severe (Red Zone) dysplasia (AI> 20° & CEA< 10°).