To search for a possibility to utilize PbWO 4 -based scintillators in inexpensive positron emission computed tomography, we have studied the effects of doping PbWO 4 with different ions on the light yield (LY). The LY in PbWO 4 , which is undoped except a small concentration of rare-earth 3+ ions to improve the radiation hardness, decay time, mechanical quality, etc., is 25-35 photoelectrons/MeV (pe/MeV) (about 3-4% of LY in BGO) with bialkali photomultiplier for a fixed condition of the crsytal size of 10x10x(20-30)mm 3 and the gate width of 1μs. For doping with single dopant, the maximum LY obtained was about 49pe/MeV for Mo 6 + . For co-doping with two dopants, the maximum LY of 58pe/MeV was obtained for Mo 6 + +Nb 5 + . For co-doping with three dopants, we have recently obtained 77pe/MeV for Mo 6 + +Cd 2 + +Sb 5 + , 3 + . The dependence of LY on the gate width indicates creation of medium-speed component in μs range in the samples doped with Mo 6 + +Cd 2 + +Sb 5 + , 3 + or Mo 6 + +Nb 5 + . Their radioluminescence spectra are similar in shape to PWO:Mo 6 + .