This paper presents the use of diffuse optical tomography (DOT) to image the human breast in vivo, using indocyanine green (ICG) as a contrast agent. The DOT images were acquired concurrently with magnetic resonance imaging (MRI) on patients scheduled for excisional biopsy or surgery. The images were accurately coregistered with MRI images, validating the ability of DOT to image breast tissue. DOT provided localization and quantification of exogenous tissue chromophore concentrations, demonstrating the potential to differentiate disease based on the quantified enhancement of suspicious lesions. The study included three cases: a ductal carcinoma, a fibroadenoma, and a control study with no suspicious enhancement. The ICG-enhanced DOT images showed good congruence with the Gd-enhanced MRI images, with the carcinoma showing a marked absorption increase and higher contrast compared to the fibroadenoma. The findings suggest that ICG could be useful in DOT mammography for cancer detection and differentiation.This paper presents the use of diffuse optical tomography (DOT) to image the human breast in vivo, using indocyanine green (ICG) as a contrast agent. The DOT images were acquired concurrently with magnetic resonance imaging (MRI) on patients scheduled for excisional biopsy or surgery. The images were accurately coregistered with MRI images, validating the ability of DOT to image breast tissue. DOT provided localization and quantification of exogenous tissue chromophore concentrations, demonstrating the potential to differentiate disease based on the quantified enhancement of suspicious lesions. The study included three cases: a ductal carcinoma, a fibroadenoma, and a control study with no suspicious enhancement. The ICG-enhanced DOT images showed good congruence with the Gd-enhanced MRI images, with the carcinoma showing a marked absorption increase and higher contrast compared to the fibroadenoma. The findings suggest that ICG could be useful in DOT mammography for cancer detection and differentiation.