Abstract: A clinical prototype of the laser optoacoustic imaging system (LOIS) for breast cancer detection with 32-element transducer array is described. The frequency band of transducer array provided 0.5-mm axial in-depth resolution. Cylindrical shape of the 10-cm long array enabled us to receive the lateral resolution of 1.0 mm in the vicinity of focal zone. The system sensitivity provided detection of a 2-mm absorbing sphere at the depth of 60mm. An automatic recognition of the optoacoustic signal detected from the irradiated surface was implemented in order to visualize the breast surface and improve the accuracy of tumor localization. Radial back-projection algorithm was used for image reconstruction. Optimal filtering of image was employed to reduce low and high frequency noise. Time necessary for data acquisition and image reconstruction was reduced to 32 sec. The system performance was evaluated initially by imaging of the small absorbing objects in breast tissue-like phantoms. Clinical ex-vivo studies of mastectomy specimens were performed and compared with X-ray radiography and ultrasound.