This article presents a new method for determining particle size distribution in undiluted suspensions from spectral measurement of transport-scattering coefficients using continuous-wave photon migration techniques. Particle size distribution (PSD) for concentrated suspensions is determined by a regularized inverse algorithm which makes it possible to incorporate a synthesized scheme of Marquardt and Tikhonov regularizations. Spectral measurements of dense TiO2 suspensions with three different particle concentrations were used to recover the suspensions＇ size distributions. Our results are in excellent agreement with the size distribution measured with X-ray sedimentation. These reconstructions of PSDs are obtained both with and without a priori distribution function assumptions.