Objectives: The objective of this study was to determine the minimal clinical important difference (MCID) and patient acceptable symptomatic state (PASS) for the modified Harris Hip Score (mHHS) and Hip Outcome Score (HOS) in a population of patients with femoroacetabular impingement treated with arthroscopic surgery of the hip. Methods: We conducted a prospective cohort study at a single centre in a consecutive series of patients with femoroacetabular impingement who were treated with arthroscopic labral repair/re-fixation and femoral osteoplasty. The mHHS and HOS were administered at baseline and at 3, 6, and 12 months post-operatively. Using external anchor questions at the aforementioned post-operative time points, the approach outlined by Juniper et al. was used to determine the MCID, whereas the specific anchor question designed by Tubach was used to determine the PASS. A receiver-operator curve was constructed and used to determine the cuff off point which optimally defined the MCID based on sensitivity and specificity values for each observed change score. The PASS was defined as the 75th percentile of the final mHHS score or HOS subscale scores for patients who considered their state satisfactory. We also stratified the analysis according whether baseline scores influenced the likelihood of achieving the MCID and PASS and odds ratios (OR) were also calculated. Results: There were 130 patients with a mean age of 35.6 (sd 11.7) years and 42.3% were male. The MCID for the mHHS at 3, 6, and 12 months was 13.0, 9.0, and 20.0, respectively. For the HOS (ADL), the MCID was 14.7, 15.0, and 23.0 at 3, 6, and 12 months. For the HOS (Sports), MCID values at 3, 6, and 12 months were 25, 28, and 47. In regards to the PASS, the observed values at 1 year following surgery were as follows: mHHS 84, HOS (ADL) 98, HOS (Sport) 94. The high values observed for the PASS across the different subscales suggests a high proportion of ceiling effects with the mHHS and HOS in this population. The MCID was significantly greater at 1 year for patients with an initial ‘low’ baseline score compared to patients with a ‘high’ baseline score. The PASS was not affected by baseline scores across different instruments. Patients with lower baseline scores were less likely to achieve the MCID one year following surgical intervention [mHHS - OR 0.28; HOS (ADL) - OR 0.23; HOS (Sports) - OR 0.06] yet more likely to achieve the PASS [mHHS - OR 3.36; HOS (ADL) - OR 3.83; HOS (Sports) - OR 3.38). There was a significant negative correlation between increasing age and the ability to obtain a mHHS or HOS above the PASS threshold (p<0.05). Age and sex were not significantly related to the odds of achieving the MCID for the mHHS or HOS. Conclusion: This is the first study in the literature to determine the MCID and PASS for two commonly used hip-joint specific patient-reported outcome measures in patients undergoing arthroscopic hip surgery for FAI. Our findings will allow researchers to determine if interventions related to FAI are meaningful to patients at the individual level across various domains and will also be useful for power calculations in future randomized trials related to hip arthroscopy and the treatment of FAI.