We investigated the long-term changes in the Harris Hip and Knee Society scores (HSS and KSS) to determine whether they result from overall functional decline rather than actual changes in the condition of the prosthesis. The HHS for 106 total hip arthroplasties with a minimum follow-up of ten years, no medical complications after operation and no evidence of radiological loosening, and the KSS for 264 total knee arthroplasties with a minimum follow-up of 12 years and no medical complications after operation or signs of radiographical loosening were evaluated. There were statistically significant drops in the functional scoring components of the joint evaluation systems despite no loosening of the prostheses or other significant medical complications. The HHS declined at an average of 0.67 points per year from between three and ten years after operation (p < 0.0001). Contributing to this were deterioration in gait and limp (p < 0.0004), the use of support aids (p < 0.0001), the distance walked (p < 0.0001) and the ability to climb stairs (p < 0.0455). The functional component of the KSS declined significantly at an average 0.88 points per year betwen the third and 12th years (p < 0.0001). There were significant declines in every component of the functional score including the distance walked (p < 0.0001), the ability to climb stairs (p < 0.0001) and the use of support aids (p < 0.0001). The knee score component of the KSS did not decline significantly (p < 0.9750). The combination of functional and pain scores within the HHS system leads to an inaccurate decline in the entire score. The decline of HHS and Knee Society functional scores in total joint arthroplasties, in the absence of implant-related problems, suggests that deterioration in the functional capacity of ageing patients is an important factor in longitudinal studies using these scoring systems