Semiconductor detectors, such as Cadmium Zinc Telluride (CZT), offer improved energy resolution (ER) and potentially improved tumor contrast in breast imaging. We evaluated the effect of changes in ER on tumor contrast. Studies were performed on a prototype CZT detector, a commercial CZT-based system, a multicrystal CsI system and a conventional NaI system. The same low energy high resolution collimator was used on each system and the same total counts (250 kcts) were acquired in each image. CZ-Tmodules in a detector can exhibit different ERs. We imaged a breast phantom containing 4 tumors (diameters 4.9, 7.2, 7.2, and 9.8 mm) over individual CZT modules with intrinsic ers of 5.8-17.5%. Tumor to background ratio was 6.2:1 and tumors were placed 1 and 4 cm deep in the 6-cm thick breast phantom. A standard +/-10% energy window and narrower energy windows (-5%/+10% and -7%/ +10%) were used. Tumor contrast was determined by maximum tumor intensity divided by mean background intensity. Tumor contrast was not significantly affected by differences in ER between CZT modules. Tumor contrast improved by 2-10% with a narrow energy window. At the same ER, the CZT systems yielded slightly better contrast than the CsI system. All pixilated systems performed better than the NaI system. Scatter in the breast was found to be low and accounts for only approximately 11% of counts in a +/-10% energy window for a 6 cm- thick breast. No strong relationship was observed between ER and tumor detectability, which may be due to low scatter in the breast. Detector element size in pixilated systems appears to be more important than ER in determining tumor contrast.