The Swedish Two-County Trial is a randomized controlled study of invitation to breast cancer screening. It was initiated in late 1977. The follow-up to the end of 1998 provides results at approximately the twentieth anniversary of the trial. A significant decrease in breast cancer death among women invited to screening was published 7-8 years after randomization and at 20-year follow up there is a significant 32% reduction in mortality associated with invitation to screening. The advent of

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... n-film mammographic screening with the ability to detect potentially fatal tumors at an early stage provides an opportunity to study the natural history of breast cancer at an earlier phase in its development than was possible in the past. Our findings show that breast cancer is not a systemic disease at its inception, but is a progressive disease and its development can be arrested by screening. Detection of < 15 mm and lymph node negative invasive tumors will save lives and confer an opportunity for less radical treatment.