We have been developing an optical mammography prototype consisting of a multi-channel time-resolved spectroscopy system for breast cancer screening. The system utilizes the time-correlated single photon counting method, and the detector modules and the signal processing circuits were custom-made to obtain a high signal to noise ratio and high temperature stability with a high temporal resolution. Pulsed light generated by a Ti: Sapphire laser was irradiated to the breast, and the transmitted light was collected by optical fibers placed on the surface of a hemispherical gantry filled with an optical matching fluid. To reconstruct a 3D image of the breast, we employed a method using a time-resolved photon path distribution based on the assumption that scattering and absorption are independent of each other. We verified the possibility of human breast imaging by using a three-dimensional phantom model, which provides a simulation of human breast cancer, in the gantry. The clinical study was also started in January 2007. In a comparative study with conventional modalities, the breast cancers were detected as regions of optically higher absorption. Moreover, the results suggest that optical mammography is useful in monitoring the effects of chemotherapy.