Pain after total knee arthroplasty is a common clinical problem in orthopedics, and prosthetic loosening, often requiring surgical revision, is usually the etiology. Since standard clinical and radiographic diagnostic measures have not proven totally satisfactory, a study of the utility of bone scintigraphy to assess stability of the knee prosthesis was done. Thirty-five patients with 39 prostheses were studied. Seventeen patients with 21 total knee arthroplasties served as controls and were asymptomatic, were stable at surgery, or improved with conservative management. Eighteen knees in 18 symptomatic patients composed the experimental group. Of these, 11 knees were loose at surgery and seven have had surgery recommended. Scintigrams of the knees were obtained using 99mTc-MDP, and ranked 0-3 corresponding to increasingly abnormal localization by three observers. Highly significant differences were observed between the abnormal and control groups (p less than 0.001). Reciprocal changes in sensitivity and specificity with increasingly stringent criteria were shown. While it is apparent that the bone scan cannot be used as the sole diagnostic method for evaluation of prosthetic stability, it does seem to be useful adjunct along with clinical criteria and radiographic studies.