Abstract: An instrument for characterizing tumor tissue includes a broad-band light source, a monochromator for filtering collimated light from the broad-band light source, a scanner for scanning the monochromator through a range of predetermined wavelengths, and a hollow needle including a shaft with a tip on one end and a base on the other end. An optical fiber is positioned within the hollow needle for delivering light from the monochromator through the tip to a desired tissue region, A photodiode is mounted in the shaft and has a light sensitive surface facing outward from the shaft for detecting back-scattered light from the tissue region. The back-scattered light detected by the photodiode is monitored by a computer which records the optical absorption spectrum of light back-scattered from the tissue region at discrete locations in the tissue as the needle is inserted or withdrawn through the mammographically abnormal tissue.