High flexion designs are intended to provide a greater range of knee flexion and possibly improve flexion in stiff knees. This study assessed the effects of two implant designs. A posterior stabilised high flexion mobile bearing (MB) design vs a cruciate retaining standard fixed bearing (FB) design. The aim of this study was to assess whether implant design has an effect on the functional outcome one year after total knee arthroplasty (TKA). Methods. Ninety patients with knee osteoarthritis on the waiting list for unilateral TKA were recruited and randomly allocated to either the MB or FB group. Patients were assessed between one and four weeks before, and one year after TKA. Primary outcome was knee flexion during high flexion activities of daily living such as stair ascending and descending and squatting as measured using gait analysis. Knee flexion in long sitting using a manual goniometer and the WOMAC were also recorded. Two sample t-tests were used to investigate statistical differences between the two groups pre- and postoperatively. Results. Average age was 69 years. Thirty-three received the MB design and 39 the FB design. Age, gender balance and pre-operative flexion (112 and 113 degrees in the FB and MB groups respectively) were the same in both groups. There were no statistically significant differences in post-operative knee flexion during functional activities. Knee flexion in sitting and the stiffness and function components of the WOMAC were also similar between the two groups (p>0.05). However, post-operatively the WOMAC pain component was slightly higher in the MB group (4.2 vs 2.4 points, p<0.05). Conclusion. In our patient group with a mean pre-operative flexion of 112.7 degrees, the high flexion mobile bearing design did not improve knee flexion during high flexion functional activities of daily living