More than 80 million aspirin (acetyl salicylic acid) tablets are consumed each day in the United States to control fever and pain and for the reduction in mortality from cardiovascular disease and possibly colorectal cancer. Observational studies reporting a protective effect of aspirin on breast cancer have been mixed. Recently Terry et al [JAMA 291(20), 2433-2440,2004] have presented statistical data, supported by other epidemiologic and laboratory evidence, to bolster their case for the use of aspirin as chemopreventive agents against breast cancer. Recent comparison of mouse and human genomes indicates that humans are genetically remarkably close to the lab mouse, and that 98% of the genes in humans have an exact match to those in the mouse. These findings bolster the notion that the mouse remains an excellent model for the study of breast cancer in general. Female C3H/Lk mice develop mammary adenocarcinomas spontaneously, and have done so consistently for over 70 inbred generations in our laboratory. In 2004 we initiated a study to determine if oral administration of aspirin has any effect on the incidence of these tumors. To date, of the 49 females receiving aspirin 42 have developed the tumors. On the other hand, of the 40 controls 36 have developed the tumors. That is, the incidence of the mammary adenocarcinoma is the same in both the experimental and control mice. These results clearly demonstrate that daily ingestion of aspirin does not prevent the appearance of breast cancers in C3H/Lk mice. (Supported by a grant from the Champlain Valley Immunology Foundation.)