It is shown that diffusion-weighted imaging is possible in the human breast. Diffusion constants were measured in the breast parenchyma of four volunteers with no known breast lesions. The apparent diffusion constant of water measured in regions of interest chosen in normal human breast fibroglandular tissue was 1.64 +/- 0.19 x 10(-5) cm2/S and that measured in the area of fatty breast tissue was 0.32 +/- 0.18 x 10(-5) cm2/S. The resulting images indicate that fibroglandular tissue and fat can be clearly distinguished in diffusion-weighted as well as in absolute diffusion images of the breast. Potential future applications of this technology for the study of breast pathologies are suggested.