From 95 subjects imaged with both speed of sound and attenuation ultrasonic computed tomography (UCT), analyses were performed on 40 cases for which unequivocal clinical diagnoses were available for correlation. This paper describes the UCT image characteristics and addresses the hypothesis that carcinomas and other lesions can be detected and localized by means of simple visual criteria or lesion characteristics that are quantitative relative to those of other breast tissues in the same patient. The most useful within-patient criterion was selection of the solid mass with the highest speed of sound in either breast (12 of 12 carcinomas). Architectural asymmetry between breasts in the three types of images was a significant contributing factor in visual image interpretation in seven of the eight cancer patients in whom there were comparable images of both breasts. Solid masses were discriminated by attenuation coefficient and pulse echo criteria. Our results did not substantiate the hypothesis that the average speed of sound throughout the cancer containing breast would be higher than in the contralateral breast. These results are better than might be expected from pulse echo imaging alone on this population. However, clinical implementation probably should be deferred until the technique is made more convenient and less expensive, or more accurate with a greater promise for diagnosis of minimal cancers.