BACKGROUND: Identifying early predictors of postoperative complications is paramount in frail patients. While the American Society of Anaesthesiologists classification is subjective, the 5-factor modified frailty index is simpler and associated with promising results in predicting morbidity and mortality in various surgical specialities. METHODS: This prospective observational study assessed the comparative utility of 5-factor modified frailty index and American Society of Anaesthesiologists classification in predicting intensive care unit admission and revision surgery in 220 patients following arthroplasty with standardised anaesthesia techniques under the same group of surgeons. The 5-factor modified frailty index and American Society of Anaesthesiologists class were assessed 1 day before surgery, and postoperatively, all the patients were followed up for 30 days. RESULTS: American Society of Anaesthesiologists and 5-factor modified frailty index had a significant association with postoperative intensive care unit requirements (odds ratio = 7.5, 95% confidence interval 1.3 to 42.4 and odds ratio = 8.3, 95% confidence interval 1.6 to 17.6, respectively). Only the 5-factor modified frailty index was significantly associated (odds ratio = 5.3, 95% confidence interval 1.3 to 9.7) with revision surgery. Patients with the 5-factor modified frailty index value of 3 had the best predictive odds for postoperative intensive care unit admission (odds ratio = 73.6, 95% confidence interval 7.5 to 718.8) and revision of surgery, particularly during total hip replacement (odds ratio = 54, 95% confidence interval 3.3). CONCLUSION: The 5-factor modified frailty index exhibited a more robust predictive capacity for postoperative intensive care unit requirements and the likelihood of revision surgery, particularly a 5-factor modified frailty index score of 3 in total hip replacement procedures.