The attenuation characteristics of a polyenergetic x-ray beam in computed tomography can be completely specified by two parameters: the spectral factor, which in turn provides an equivalent single energy, and the noise factor, which describes the noise characteristics of the polyenergetic beam relative to its single-energy equivalent. Noise factors of typical spectra have been calculated. It is found that only moderate filtration is required to bring the noise factors near unity, indicating that there is little increase in image noise caused by the use of polyenergetic spectra. By using noise factor as an analytic tool, it is possible to optimise the filtration for the desired adjustment between noise, scan time, and dose.