The use of an adjunctive modality to radiographic mammography is one way of improving the effectiveness of a breast cancer screening program. However, before introducing such a procedure into a screening program, an objective evaluation process must be developed to establish adequate guidelines to determine system performance. The success of the clinical evaluation of a diagnostic system is as much a factor of the performance of the system as it is a factor of the interpretation of system data. The Phase I program proved the feasibility of using a multiple antenna microwave radiometry system for detecting """"""""hot spots"""""""" embedded within a two-layer phantom model representative of biological tissue. The response of the radiometric system compared favorably with predicted results of a mathematical model describing the phantom. A technique, developed to evaluate the response of an antenna or antenna array to """"""""hot spots"""""""" embedded within the phantom, yields a figure of merit descriptive of the spatial resolution of the antenna configuration. The intention of the Phase II program is to use the phantom model data to optimize the multiple antenna array for detecting abnormal thermal activity in breast tissue and then evaluate the resultant system on volunteers. Compression plates, housing the antennas, will be used to reduce tissue thickness and provide thermal views of the breast from opposing surfaces. Data measured on volunteers will be compared with mammographic, palpation and physical findings to determine the sensitivity, specificity and accuracy of the system.