Alteration of knee alignment after unicompartmental knee arthroplasty (UKA) influences wear of the prosthesis and progression of arthrosis. Recent reports have questioned the traditional view that under-correction of the deformity is advisable in UKA. The aim of this study was to analyse whether the location of the mechanical axis at the knee influences the function of the knee after UKA. We analysed the data from 40 patients (54 knees) who underwent UKA over 17 years. The Bristol knee score was maximal when the mechanical axis passed through zones 0 (area of tibial spines) or 1 (inner half of medial or lateral plateau). The average Bristol knee score of those patients was 18% higher than in patients in whom the mechanical axis passed through the other zones (p < 0.01, t-test). Using multiple regression analysis to correct for the effect of age on score, a significant relationship was found between the location of mechanical axis and function (p < 0.001). Based on these findings, surgeons should attempt to restore the mechanical axis to the centre of the knee during UKA, to help achieve better function.