Background: Postoperative delirium (PD) is a well-known complication among elderly surgical patients and associated with increased morbidity, mortality and length of stay (LOS). In elective orthopedic surgery, including hip and knee arthroplasty (THA/TKA), most studies report incidences between 5% and 10%. The multimodal optimization of perioperative care (fast-track) aims to enhance recovery and reduce morbidity and LOS, but limited data are available on the effect on PD. Consequently, the study investigated signs of PD associated with LOS > 4 days. Methods: Prospective risk assessment study with retrospective analysis of discharge notes or medical records of signs of PD in 6331 elective primary unilateral THA and TKA patients ≥ 70 years, and LOS > 4 days. Preoperative patient characteristics collected from eight high volume centers with similar standardized fast-track protocols from January 2010 to November 2013. Results: We identified 43 (0.7%) cases of PD symptoms mentioned as a reason for LOS > 4 days among the 789 patients with LOS > 4 days (12.5% of all THA and TKA). PD patients had a mean age of 80.7 [[95% CI] 79.3–82.1] years, being 4.0 [[95% CI] 2.5–5.5] years older compared to patients without PD (P < 0.001). LOS was median 10 [[Q2–Q3] 7–14] days in the PD group vs. 3 [2–3] days in the non-PD group (P < 0.001), without differences in gender or site of arthroplasty (P = 0.139 and 0.499, respectively). Conclusion: Postoperative delirium symptoms contributing to LOS > 4 days in fast-track THA and TKA are rare in elderly patients.