The image data from 348 breast CT studies performed on patient volunteers in a Phase II clinical trial were used in a pre-whitened matched filter (PWMF) model observer study. This computer-based investigation simulated the addition of spherical lesions of various diameters to the breast data sets, and a series of images with different slice thicknesses ranging from 0.3 mm to 44 mm were created by voxel averaging. The PWMF was tailored to each slice thickness, and signal known exactly receiver operating characteristic analysis was performed. A total of 1000 lesions and "non-lesions" were simulated for each diameter on each breast data set. Receiver operating characteristic (ROC) curve analysis was performed, and sensitivity at 95% specificity was computed. Thin slice imaging as with breast CT demonstrated significantly better sensitivity than thick slice images as with mammography, with improvements in sensitivity ranging from 5% to 50%.