Diffuse optical spectroscopy (DOS) and imaging are emerging diagnostic techniques that quantitatively measure the concentration of deoxy-hemoglobin (ctHHb), oxy-hemoglobin (ctO2Hb), water (ctH2O), and lipid in cm-thick tissues. In early-stage clinical studies, diffuse optical imaging and DOS have been used to characterize breast tumor biochemical composition and monitor therapeutic response in stage II/III neoadjuvant chemotherapy patients. We investigated whether DOS measurements obtained before and 1 week into a 3-month adriamycin/cytoxan neoadjuvant chemotherapy regimen can predict final, postsurgical pathological response. Baseline DOS measurements of 11 patients before therapy revealed significant increases in tumor ctHHb, ctO2Hb, ctH2O, and spectral scattering slope, and decreases in bulk lipids, relative to normal breast tissue. Tumor concentrations of ctHHb, ctO2Hb, and ctH2O dropped 27 ± 15%, 33 ± 7%, and 11 ± 15%, respectively, within 1 week (6.5 ± 1.4 days) of the first treatment for pathology-confirmed responders (n = 6), whereas nonresponders (n = 5) and normal side controls showed no significant changes in these parameters. The best single predictor of therapeutic response 1 week posttreatment was ctHHb (83% sensitivity, 100% specificity), while discrimination analysis based on combined ctHHb and ctH2O changes classified responders vs. nonresponders with 100% sensitivity and specificity. In addition, the pretreatment tumor-to-normal ctO2Hb ratio was significantly higher in responders (2.82 ± 0.44) vs. nonresponders (1.82 ± 0.49). These results highlight DOS sensitivity to tumor cellular metabolism and biochemical composition and demonstrate its potential for predicting and monitoring an individual's response to treatment. © 2007 by The National Academy of Sciences of the USA.