Abstract: Therapeutic methods for preventing or retarding organ transplant rejection are described, the methods including delivering to a transplanted organ a rejection effective amount of light energy, the light energy having a wavelength in the visible to near-infrared wavelength range, wherein delivering the rejection effective amount of light energy includes selecting a power density (mW/cm2) of light energy to be received at the organ. The power density is at least about 0.01 mW/cm2 and no more than about 100 mW/cm2, to be delivered to the transplanted organ after completion of the transplant procedure.