BACKGROUND: Recent studies have noted that a large fraction of elderly patients do not receive conventional treatment for breast cancer. The consequences of undertreatment of the elderly have not been adequately assessed. STUDY DESIGN: The senior author's database (PIT) was used to identify women undergoing potentially curative operations for breast cancer between 1978 and 1998. Risk factors, presentation, pathologic findings, treatment, and outcomes of 206 women aged over 70 years were compared with those of 920 younger patients. In addition, conventionally treated and "undertreated" elderly patients were identified, and their characteristics and outcomes were compared. RESULTS: Older patients' cancers were more often visible on mammography, usually as a mass; younger patients' mammograms were less frequently positive, presenting more often with calcifications (p = 0.002). Cancers of the elderly were better differentiated (p < 0.001) and more likely to be estrogen- and progesterone-receptor positive (p < 0.001; p = 0.007). Patients over 70 had fewer mastectomies (19% versus 33%; p < 0.001) and were also less likely to undergo axillary node dissection (71% versus 81%, p = 0.006), postoperative radiation (69% versus 92%, p < 0.001), and chemotherapy (18% versus 48%, p < 0.001). Fifty-seven percent of older patients were treated with tamoxifen compared with 36% of younger patients (p < 0.001). Elderly patients' rates of local and distant recurrence were comparable to those of younger patients after both mastectomy and breast conservation. Ninety-eight patients (54%) over 70 were undertreated by conventional criteria. Undertreated elderly patients were significantly older (78 versus 76 years, p = 0.003), were diagnosed with excisional biopsy more often (69% versus 57%, p = 0.069) and with fine-needle aspiration less frequently (22% versus 38%, p = 0.069), and were more likely to have breast conservation (90% versus 73%, p = 0.004). Local and distant disease-free survival rates of both groups were comparable. Tamoxifen treatment significantly reduced the chance of developing distant metastasis in node-negative elderly patients with invasive tumors (p = 0.028). Omission of chemotherapy had no impact on disease control in the elderly. Axillary node status and estrogen-receptor status were significantly related to local disease-free survival, and axillary node status was significantly related to distant disease-free survival in multivariate analysis in the elderly. CONCLUSIONS: Elderly breast cancer patients are frequently treated with breast conservation, omitting axillary dissection, radiation therapy, and chemotherapy. Despite undertreatment by conventional criteria, the rates of local recurrence and distant metastasis are not increased in comparison with conventionally treated elderly patients. Tamoxifen should be administered to elderly breast cancer patients with invasive tumors because it significantly improves distant control.