To identify factors predicting failure after hip arthroscopy in patients with previous periacetabular osteotomy (PAO) defined as a conversion to total hip replacement (THR) and to evaluate the patient reported outcome scores. Of 55 hips treated with hip arthroscopy after PAO from Aug 2008 to 2012 at Aarhus University Hospital, 43 hips were included (median age: 36.1 yrs, range 16.3-56.9 yrs). Indications were unacceptable pain, a positive FABER and impingement test and signs of labral damage on MR-arthrography. Outcomes were evaluated with mHHS and HOS. Failure was defined as conversion to a THR. Nine hips were converted to a THR. Kaplan-Meier survival rate was 52.8% (95% CI, 10%-83.8%) at 6.5 years follow-up. Statistically significant predictors of failure: joint space width after PAO <3.0 mm and Tönnis grade of 2. Fourteen hips needed revision hip arthroscopy. Labral damage was present in 84% of the hips. In 42% of the hips cartilage lesions of Becks grade >3 were found. Mean mHHS and HOS were 65.7 and 68.8 respectively at follow-up. A NRS pain score of >3 in rest and during activity were present in respectively, 43% and 62% of the patients. Hip arthroscopy after PAO demonstrated limited clinical benefit with no decrease in pain levels and 21% of patients needing reoperation to THR. Radiographic signs of joint degeneration after PAO are predictors of faiElure. Further studies are needed to clarify what role hip arthroscopy should play in this patient group.