An instrument for time-resolved optical mammography was constructed and is regularly used in an ongoing clinical trial. The system operates with four pulsed diode lasers emitting at 683, 785, 913, and 975 nm. The breast is slightly compressed between two Plexiglas plates and scanned along the X and Y directions. Light transmitted through the breast is collected by a 5 mm diameter optical bundle and coupled into two compact photomultipliers with enhanced sensitivity in the visible and near infrared spectral region, respectively. A multichannel PC board for time-correlated single photon counting is used for signal processing. Continuous acquisition and on line display of the processed images permit optimal measurement efficiency and control of the operation. A typical optical examination complies measurements obtained on both breasts in craniocaudal and oblique projections, with a typical measurement time of 5 minutes for each scan. In this paper we demonstrate the feasibility of in vivo time-resolved optical mammograms acquired at wavelengths longer than 900 nm in the lipid and water absorption region. The results of a clinical trial performed on 101 patients are shown, demonstrating capability to discriminate between benign and malignant breast lesions on the basis of different spectral behaviors. Furthermore, a non-linear perturbation model was successfully applied to the in vivo characterization of breast lesions. Preliminary results on tissue constituent determination (in term of blood volume, blood oxygen saturation, lipids and water content) were presented.