A technique for determining the local drop size distribution in liquid sprays is developed. This technique is based on the far-field measurement of the single-wavelength back-scattered light from the spray. A combination of a tomographic imaging technique and the Mie scattering theory is used. The tomographic reconstruction of the spray is realized by using several optical projections, which are obtained by recording the back-scattered light when the spray is illuminated with a coherent and collimated laser beam. Each of these projections corresponds to a specific angular orientation of the spray and a single back-scattering direction. Such an image represents the local lime-averaged intensity across the spray for the light scattered in the selected direction. Three tomographic images of the same cross section, corresponding to three different back-scattering directions, are used to determine the local drop size distributions. A log-normal distribution function defined by two variables is assumed, and the scattered light intensity by this distribution is calculated from the Mie theory for the three back-scattering directions. A discrete search technique is then implemented to find the local log-normal distribution that best matches the results provided by the Mie theory and the tomographic images for all three directions. Simulations realized with an axisymmetric spray underline the accuracy of this new method.