We studied the complications involving the patella following total knee arthroplasty in eighty-six knees in which thirty-four unconstrained and fifty-two offset hinge prostheses had been implanted. The abnormalities that we studied included: patellar dislocation, five knees; subluxation, eighteen knees; localized wear, three knees; and generalized wear, four knees. In twenty-three patients these complications were associated with patellar malalignment, and occurred predominantly when the offset hinge model of prosthesis was used. The complications were attributed partly to mechanical factors inherent in the prosthetic design and partly to anatomical abnormalities. Failure of the surgeon to compensate adequately for both of those factors at the time of operation was a factor in most of the complications.