Introduction. The enhanced recovery after surgery (ERAS) concept in arthroplasty surgery has led to a reduction in postoperative length of stay in recent years. Patients with prolonged length of stay (PLOS) add to the burden of a strained NHS. Our aim was to identify the main reasons. Methods. A PLOS was arbitrarily defined as an inpatient hospital stay of four days or longer from admission date. A total of 2,000 consecutive arthroplasty patients between September 2017 and July 2018 were reviewed. Of these, 1,878 patients were included after exclusion criteria were applied. Notes for 524 PLOS patients were audited to determine predominant reasons for PLOS. Results. The mean total length of stay was 4 days (1 to 42). The top three reasons for PLOS were social services, day-before-surgery admission, and slow to mobilize. Social services accounted for 1,224 excess bed days, almost half (49.2%, 1,224/2,489) of the sum of excess bed days. Conclusion. A preadmission discharge plan, plus day of surgery admission and mobilization on the day of surgery, would have the potential to significantly reduce length of stay without compromising patient care. Cite this article: Bone Joint Open 2020;1-8:488–493