Research suggests that there exists a significant contrast between the dielectric properties of cancerous and normal breast tissue at microwave frequencies. Open-ended coaxial probes have been extensively used and analyzed in dielectric spectroscopy. There has been an extensive numerical and experimental analysis of these probes to ensure the highest possible accuracy in the dielectric-property estimates. For simple probes made of sections of semi-rigid cable, small geometrical changes at the probe aperture, which can result from manufacturing, handling and/or usage, may have very significant effects on the observed reflection coefficient. Consequently, a precision, 3.0 mm, stainless steel open-ended coaxial probe has been designed. The probe is built out of sections of high quality coaxial line, using dielectrics whose thermal properties are similar to those of the metal. Tolerances on probe aperture dimensions and flatness are better than 10 /spl mu/m. Borosilicate glass, hermetically sealed to the metal conductors, is used as the dielectric in the final section of the probe. We present preliminary experimental and numerical reflection coefficient estimates using this probe. The carefully manufactured open-ended coaxial probe is reliable and yields accurate results, and is suitable as a sensor for high fidelity in vivo tissue measurements and characterization.