Optoacoustic imaging is a promising new tool for the detection and diagnosis of breast cancer. It is progressing from research study to clinical evaluation. We have now built a complete laser optoacoustic imaging system (LOISTM) consisting of a laser illumination system, a 32-element ultrasonic detector probe, signal amplifiers, and a computer with software for image generation. This report describes initial tests to explore the clinical viability of the system. Our results show that the system has sufficient sensitivity to reveal cancerous tumors already identified with X-ray and/or ultrasound imaging, that it has the resolution to show faithfully the size and shape of those tumors, that comparison of images taken at 755 and 1064 nm is indicative of whether or not a suspicious lesion is cancerous, and that the depth of sensitivity of the system is sufficient to detect tumors throughout an average-sized breast.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.