Sensitivity to a series of haptens was studied in two groups of patients by means of patch tests. The first group was made up of 66 patients with total hip replacements, 12 of whom had aseptic loosening of the implant. The second group, which was used as a control, consisted of 41 patients scheduled to undergo total hip replacement. Ten patients (15.1%) in the first group tested positive; one of these had aseptic loosening. In the control group, two patients (4.8%) tested positive. The theory that an allergy may be the cause of loosening has a biologic foundation, but it can only be used to explain a few cases. However, since considerable vanadium sensitivity was found, we suggest that this material be used as little as possible in the manufacturing of prosthetic components.