Previous investigation has proved time-resolved spectroscopy to be applicable to measurement of optical parameters in the human breast. To increase knowledge of these properties in vivo, the optical parameters of healthy breasts were measured using time-resolved reflectance spectroscopy. A time correlated single-photon counting method was used to obtain time-response curves for the breasts of 30 Japanese women. Values of ma (absorption coefficient) and ms8 (transport scattering coefficient) were analyzed by fitting the curves to the diffusion equation. The relationships of optical parameters to age, body mass index (BMI), thickness of the breast, number of pregnancies, and menstrual status were examined. The ma and ms8 ranged from 0.0024 to 0.0078/mm and from 0.63 to 1.08/mm, respectively. The values of ma and ms8 showed a high correlation with
age and BMI, respectively. The range of the optical parameters of the healthy breasts was determined. These properties may be strongly influenced by changes in tissue components related to aging, menstrual status, and so on. This optical information will contribute to the investigation of photon migration in the human breast.