Until the 1960s, the central public health message about breast cancer was that women should not delay seeking medical attention for breast problems. Epidemiological, pathological, public health, and clinical writings, movies, and doctor-patient correspondence are analyzed in order to understand the durability and centrality of this “do not delay” message. Problematic assumptions about the natural history of cancer, the efficacy of surgery, and individual responsibility for disease contributed to the durability of the “do not delay” message. More important, the message catalyzed or sustained changes in the routines of ordinary women, general practitioners, surgeons, and pathologists, which led to the perception that the campaign against cancer was working. Thus a powerful set of reinforcing perceptions and behaviors maintained the centrality of the “do not delay” campaign until the era of mammography.