An assessment was made of the roles of 99Tcm-MDP, 67Ga-citrate and 111In-leucocytes in identifying infected hip prostheses. Fifty painful prosthetic hips were investigated with a 99Tcm-MDP bone scan and an 111In-labelled leucocyte scan (ILLS). In 32 cases, a 67Ga-citrate scan was also performed. Normal or focal MDP uptake was obtained only in uninfected hips. Diffuse uptake of MDP was obtained only in infected hips and in one case of non-septic synovitis which also produced a false positive ILLS. A focal MDP uptake superimposed on a diffuse pattern was obtained in both infected and uninfected hips. The ILLS was abnormal in eight out of 11 cases of infection and in two out of 39 cases without infection. The 67Ga scan was abnormal in five out of six cases of infection and in five out of 26 cases without infection. It was concluded that a 99Tcm bone scan should be performed first on a painful prosthetic hip, and that for detecting infection, an ILLS was more specific but less sensitive than a 67Ga-citrate scan.