The National Surgical Adjuvant Breast and Bowel Project (NSABP) conducted two sequential randomized clinical trials to aid in resolving uncertainty about the treatment of women with small, localized, mammographically detected ductal carcinoma in situ (DCIS). After removal of the tumor and normal breast tissue so that specimen margins were histologically tumor-free (lumpectomy), 818 patients in the B-17 trial were randomly assigned to receive either radiation therapy to the ipsilateral breast or no radiation therapy. B-24, the second study, which involved 1,804 women, tested the hypothesis that, in DCIS patients with or without positive tumor specimen margins, lumpectomy, radiation, and tamoxifen (TAM) would be more effective than lumpectomy, radiation, and placebo in preventing invasive and noninvasive ipsilateral breast tumor recurrences (IBTRs), contralateral breast tumors (CBTs), and tumors at metastatic sites. The findings in this report continue to demonstrate through 12 years of follow-up that radiation after lumpectomy reduces the incidence rate of all IBTRs by 58%. They also demonstrate that the administration of TAM after lumpectomy and radiation therapy results in a significant decrease in the rate of all breast cancer events, particularly in invasive cancer. The findings from the B-17 and B-24 studies are related to those from the NSABP prevention (P-1) trial, which demonstrated a 50% reduction in the risk of invasive cancer in women with a history of atypical ductal hyperplasia (ADH) or lobular carcinoma in situ (LCIS) and a reduction in the incidence of both DCIS and LCIS in women without a history of those tumors. The B-17 findings demonstrated that patients treated with lumpectomy alone were at greater risk for invasive cancer than were women in P-1 who had a history of ADH or LCIS and who received no radiation therapy or TAM. Although women who received radiation benefited from that therapy, they remained at higher risk for invasive cancer than women in P-1 who had a history of LCIS and who received placebo or TAM. Thus, if it is accepted from the P-1 findings that women at increased risk for invasive cancer are candidates for an intervention such as TAM, then it would seem that women with a history of DCIS should also be considered for such therapy in addition to radiation therapy. That statement does not imply that, as a result of the findings presented here, all DCIS patients should receive radiation and TAM. It does suggest, however, that, in the treatment of DCIS, the appropriate use of current and better therapeutic agents that become available could diminish the significance of breast cancer as a public health problem.