Abstract: A data reduction method is presented for three-dimensional ultrasound computer tomography. This method identifies the earliest surface reflection of the breast, reconstructs the breast surface, calculates the begin and end time of the echo signals from the breast, builds time windows to mark the range of the valid data for imaging algorithms in the sequences of samples and extracts useful data to complete the data reduction. 15 breasts with different shapes and mislocalizations are used to evaluate the limitations of the technique. Based on clinical data, the presented method achieves a mean compression ratio of 2.27, a standard deviation of 0.21 and a speedup factor of 2.25 for the data transmission. This method is only based on the time-domain information of echoes; therefore, it can be applied as a pre-compression method and combined with conventional methods for a higher compression ratio.