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There is scant literature on the evaluation of dislocation after total hip arthroplasty (THA) in patients with ipsilateral valgus knee deformity. This study aimed to investigate the post-operative dislocation rate in patients with valgus knee deformity who underwent ipsilateral THA and identify whether ipsilateral valgus knee deformity increases the dislocation rate after THA.We retrospectively reviewed patients with valgus knee deformity who underwent ipsilateral THA in our institution from January 2016 to December 2018. Each hip with ipsilateral valgus knee deformity was matched with a hip without valgus knee deformity according to sex, affected side, and date of surgery. The primary outcome was the dislocation rate after THA. Univariate analyses were initially used to compare data between the dislocation group and the non-dislocation group. Independent risk factors for dislocation were determined using multivariate logistic regression.There were 257 THAs with ipsilateral valgus knee deformity (valgus knee group) and 257 THAs without valgus knee deformity (control group). The valgus knee group showed a significantly higher dislocation rate than the control group (9.7% versus 1.6%, p 0.001). Older age (p = 0.020) and malposition of the acetabular cup (p = 0.048) were independent risk factors of post-operative dislocation.Patients with valgus knee deformity have a higher risk of dislocation after ipsilateral THA. If ipsilateral THA and total knee arthroplasty must be performed successively, total knee arthroplasty may have an earlier priority than THA.