Purpose. Femoroacetabular impingement syndrome (FAI) is a common cause of hip and groin pain in young adults. Physiotherapy and surgery have both been used to treat FAI syndrome, but there is no robust evidence of comparative effectiveness. UK FASHIoN compared the clinical and cost-effectiveness of arthroscopic hip surgery (HA) versus best conservative care in patients with FAI syndrome. Methods. UK FASHIoN was a pragmatic, multicentre, 2 parallel arm, superiority, randomised controlled trial in patients with FAI syndrome. Eligible patients were over 16 without radiographic signs of osteoarthritis, deemed suitable for arthroscopic FAI surgery. Participants were randomly allocated to HA or Personalised Hip Therapy (PHT - a physiotherapist-led programme comprising 6 to 10 sessions). The primary outcome measure was hip-related quality of life using the patient-reported International Hip Outcome Tool (iHOT-33) at 12 months. Secondary outcomes included EQ5D5L, SF12, adverse events, and cost-effectiveness. Primary analysis compared differences in iHOT-33 scores at 12 months by intention to treat. Results. 348 patients were randomised. Time to surgery was 132 days (SD71) versus 47 days (SD52) to PHT. 92.5% were followed-up at 12 months. Baseline mean iHOT-33 scores were 39.2 (SD21) and 35.6 (SD18) in the surgery and PHT groups, and at 12 months 58.8 (SD27) and 49.7 (SD25) respectively. Mean scores in both groups improved over 12 months, but the mean iHOT-33 score increased more in those allocated to HA than to PHT, with an adjusted mean difference of 6.8 points (95% CI 1.7,12.0 p=0.009). One HA patient developed an infection requiring further surgery. Mean overall costs were £3713 for HA and £1283 for PHT. Conclusions. Hip arthroscopy and best conservative care both led to improved hip-related quality of life in patients with FAI syndrome. At 12-month follow-up, improvement was greater in those allocated to hip arthroscopy