We have developed a clinical microwave imaging system for breast imaging applications including cancer detection and monitoring of neoadjuvant chemotherapy. For the latter, key features of any montoring system must include the ability to image a target in a repeatable fashion while being noninvasive and relatively inexpensive. Our microwave imaging system is particularly useful in this setting because we have overcome challenges such as the use of a priori information with the combination of our unique imaging configuration and software. The hardware incorporates monopole antennas which present isotropically radiated fields within which complex nulls (and accompanying non-unique signal phase distributions) are generally not generated until the electromagnetic wave has propagated beyond the illumination zone and associated receiver antennas. This is particularly useful for our variance stabilizing transformation algorithm which can recover accurate dielectric property maps without a priori information or converging to local minima as long as the unwrapped phase distribution remains unique. The associated unwrapping of the measurement data can be readily performed through the use of multi-frequency data and standard unwrapping algorithms. In this paper, we describe our clinical system and the first set of images from a patient undergoing chemotherapy. She received four doses of doxorubicin and cyclophosphamide once every three weeks and was imaged with the microwave system during her first three hospital visits. Subsequently she also received a second round of chemotherapy once every two weeks for six weeks after which we were able to image her a last time. She was also imaged with gadolinium-enhanced MR at the start of the chemotherapy and just prior to surgery. The patient was 36 years old, her breasts were considered radiographically scattered density and the tumor was estimated 6.5 to 7.0 cm in diameter prior to the start of the treatment. The microwave images clearly show a distinct reduction in the elevated property (both permittivity and conductivity) zones associated with the tumor location. These were consistent with the clinical assessment where the oncologists concluded that the tumor had responded completely to the treatment based on the MR images and on conventional CT images examining the size of the axillary adenopathy. This is an exciting result in that it demonstrates our ability to image a large, high contrast object within an in vivo, low permittivity organ and recover reliable and repeatable images. This may become an important clinical application for microwave imaging given the limited tools currently available to breast oncologists.