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Introduction The purpose of this study was to assess if severity of radiographic changes of knee arthritis was associated with patient improvement after total knee arthroplasty (TKA). We hypothesised that patients with mild arthritis were more likely to report lower satisfaction, improvement in knee function and Oxford knee score (OKS) compared to patients with moderate or severe arthritis. Materials and methods Secondary analysis of prospectively collected data from TKA patients of two arthroplasty centres with knee radiographs available for assessment of disease severity. Patients completed the Oxford knee score (OKS) and were asked to rate the global improvement in knee condition and their satisfaction at 6 months post-TKA. Bivariable analysis and multivariable regression models were used to test the association between disease severity and each outcome. Results 2226 patients underwent primary TKA and 3.6% had mild arthritis. Mean OKS improved from 17.0 (SD 18.0) to 38.0 (SD 8.1) 6 months after TKA. Two hundred and twenty-two patients (10%) reported ‘Poor’ or ‘Fair’ satisfaction, and 173 (8%) reported knee function was ‘Much worse’, ‘A little worse’ or ‘About the same’ 6 months post-TKA. Patients with mild arthritis showed improvement in OKS [mean improvement in OKS = 19 (SD 15)], but were significantly more likely to report dissatisfaction (OR = 3.10, 95% CI 1.62 to 5.91, p = 0.006), lack of improvement (OR = 4.49, 95% CI 2.38 to 8.47, p < 0.001) and lower OKS scores (− 3 points, 95% CI − 5.39 to − 0.85, p = 0.008) compared to patients with moderate to severe arthritis. Conclusions While patients with mild radiographic arthritic changes improve after TKA, they were significantly more likely to report higher rates of dissatisfaction, less improvement in knee function and OKS compared to patients with moderate-severe grades of arthritis.