This paper studies the potential of ultra-wideband (UWB) microwave imaging for detection and localization of breast cancer in its early stages. A method is proposed for locating tumors which is based on the time-of-∞ight of the signal backscattered at the tumor. Time- of-∞ight is detected using a wavelet transform algorithm. The main contribution of this paper is that it proposes to determine the position of the tumor by using an adapted version of the centroid localization method used in wireless sensor nodes. Its main advantage is that it does not require knowing a priori neither the propagation velocity of the breast nor its dielectric permittivity. The feasibility of the method has been investigated by means of simulated and experimental results with an ultra-wideband radar and a phantom.