A new impedance analytical system was developed, and measurements were performed over a frequency range of 0-200 kHz by the three-electrode method. The three electrodes consist of a coaxial needle electrode inserted into the tumor and a large reference electrode on the upper abdominal wall. The electrical bio-impedance was measured in 54 patients with breast tumors. The biological tissue can be regarded electrically as an equivalent consisting of extracellular resistance (Re), intracellular resistance (Ri), and electrical capacitance of the cell membrane (Cm). These three parameters were calculated from the measured values of electrical bio-impedance by the curve-fitting technique using a computer program. It was found that Re and Ri of breast cancers were significantly higher than those of benign tumors (p less than 0.01), and that Cm of breast cancers was significantly lower than that of benign tumors (p less than 0.01). Measurement of the electrical bio-impedance of breast tumors may have value in the differential diagnosis of breast lesions.