PURPOSE: The aim of this nationwide study was to investigate the early morbidity after bilateral simultaneous and staged total knee arthroplasty (TKA) in order to clarify potential benefits of a well-established fast-track regime. METHODS: The Danish National Patient Registry was searched for all bilateral simultaneous and staged TKA procedures from 2010 to 2011. The staged procedures were defined as two separate procedures done within 0-6 months or within 7-18 months. RESULTS: A total of 157 patients had bilateral simultaneous TKA, 346 patients had bilateral staged TKA within 0-6 months and 292 patients had bilateral staged TKA within 7-18 months. The median length of stay in hospital (LOS) was 4 days (interquartile range, IQR: 3) after bilateral simultaneous TKA versus cumulated LOS of 6 days (IQR: 3) in both of the bilateral staged groups (p < 0.001). There were no deaths after bilateral simultaneous TKA versus three deaths (0.9 and 1.0 %) in each of the bilateral staged groups within 90 days of surgery (n.s.). The total readmission rate within 30 days of surgery was lower after bilateral simultaneous TKA (7 %, CI 4.0-12.0) and bilateral staged TKA within 0-6 months (9 %, CI 6.4-12.4) compared with 14 % (CI 11.5-20.1) after bilateral staged TKA within 7-18 months. CONCLUSIONS: The results from this nationwide study indicate that bilateral simultaneous TKA can safely be performed in a fast-track set-up.