UNLABELLED: Microwave imaging has been proposed for breast cancer detection and treatment monitoring. Prototype systems based on tomography and radar-based techniques have been tested on human subjects with promising results. Previously, we developed a system that estimated average permittivity in regions of the breast using signals transmitted through the tissues. Encouraging results with volunteers and patients motivated development of a system capable of creating more detailed images of the entire breast. OBJECTIVE: In this paper, we aim to assess the performance of this next generation microwave imaging system and demonstrate scans of human subjects that relate to clinical information. METHODS: With a novel imaging system, scans of homogeneous phantoms and phantoms with inclusions of various sizes are collected. The accuracy, detection and localization are assessed. A pilot study is carried out with a small group of volunteers with previous mammograms. RESULTS: Images of flexible phantoms have average error of less than 10 % in the reconstructed average permittivity. Detection of inclusions of 1 cm diameter and greater is demonstrated. The feasibility of scanning human subjects is also demonstrated by providing microwave images of several healthy volunteers with previous mammograms. SIGNIFICANCE: A novel high-resolution microwave transmission imaging system, in conjunction with a fast quantitative reconstruction algorithm capable of detecting 1 cm diameter inclusions, is designed for breast imaging applications. It can image various breast sizes without the need for matching fluid. CONCLUSION: Overall, the results indicate that this imaging system is well suited for further exploration of microwave imaging with human subjects.