Dielectric properties of breast tissue obtained from 131 patients were investigated by an open-ended probe separated from the tissue by a dielectric film (plastic foil). The film decreases the measured capacitance, but the content of blood and body liquids has little effect on the measured quantity. We found that in cancerous tissue the electric capacity (related to the permittivity) measured at f > or = 10 MHz using the modified probe is higher than that in normal tissue. The dielectric assignment was confirmed by histopathology in 94% of investigated cases. High efficiency in differentiation between normal and cancer breast tissues and the feasibility of fulfilling of aseptic conditions (a film covering the probe) are promising in intra-operative cancer diagnostics.