Background and purpose - The incidence of manipulation under anesthesia (MUA) after knee arthroplasty surgery has been reported to vary between 0.5% and 10%. We evaluated the incidence of MUA after primary knee arthroplasty in Sweden, the demographics of the patients and the risk of revision. Patients and methods - Between 2009 and 2013, 64,840 primary total and unicompartmental knee arthroplasties (TKA and UKA) were registered in the Swedish Knee Arthroplasty Register (SKAR). MUAs performed between 2009 and 2014 were identified through the in- and outpatient registers of the Swedish National Board of Health and Welfare. Pertinent data were verified through medical records and patient demographics and revisions were obtained from the SKAR. Results - 1,258 MUAs were identified. Of these, 1,078 were 1st-time MUAs, performed within 1 year after the primary knee arthroplasty. The incidence of MUA was 1.7% and the incidence varied between hospitals from 0% to 5%. The majority were performed after TKA (98%), in younger patients (65%