The correct choice of therapy for localized breast cancer is controversial. A great many published clinical experiences, some of considerable size, have tended on the one hand to provide apparently contradictory information, and on the other hand to polarize views. A detailed consideration of the many clinical, pathologic, biochemical and kinetic prognostic variables which may bias such clinical experiences will be discussed. These may contribute to differing results from institutions apparently employing a single treatment plan. The prospectively randomized trials which have helped to establish the limits of acceptable therapy for localized breast cancer will also be reviewed together with comments on their limitations. Particular attention will be paid to National Surgical Adjuvant Breast Project (NSABP) trial B-06; the Milan Cancer Institute trial; and current National Cancer Institute (NCI) investigations. A thorough consideration of these issues should permit a more rational choice of primary treatment for patients with Stage I and Stage II breast cancer.