Aggressive malignant tumors may be diagnosed based on relative concentration of oxyhemoglobin and deoxyhemoglobin in the tumor microvasculature. Optoacoustic images of breast cancer and prostate cancer may be acquired at two laser wavelengths matching maximum of oxyhemoglobin (1064-nm, Nd:YAG laser) and deoxyhemoglobin (760-nm, Alexandrite laser). Two optoacoustic systems operating in forward and backward mode respectively for breast cancer and prostate cancer detection, employing arrays of ultravide-band piezoelectric transducers and multichannel electronics was described. After systems testing and calibration in phantoms, initial experiments were performed on patients with suspicious tumors. Quantitative analysis of two-color optoacoustic images was correlated with biopsy and histology. Possibility for tumor differentiation was demonstrated.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.