The first, to our knowledge, in-vivo broadband spectroscopic characterization of breast tissue using different interfiber distances as well as transmittance measurements is presented. Absorption and scattering properties are measured on six healthy subjects, using time-resolved diffuse spectroscopy and an inverse model based on the diffusion equation. Wavelength-tunable picosecond-pulse lasers and time-correlated single-photon counting detection are employed, enabling fully spectroscopic measurements in the range 610 to 1040 nm. Characterization of the absorption and reduced scattering coefficients of breast tissue is made with the aim of investigating individual variations, as well as variations due to different measurement geometries. Diffuse reflectance measurements at different interfiber distances (2, 3, and 4 cm) are performed, as well as measurements in transmittance mode, meaning that different sampling volumes are involved. The results show a large variation in the absorption and scattering properties depending on the subject, correlating mainly with the water versus lipid content of the breast. Intrasubject variations, due to different interfiber distances or transmittance modes, correlate with the known structures of the breast, but these variations are small compared to the subject-to-subject variation. The intrasubject variations are larger for the scattering data than the absorption data; this is consistent with different spatial localization of the measurements of these parameters, which is explained by the photon migration theory. © 2004 Society of Photo-Optical Instrumentation Engineers.