Purpose: Although 48% of breast cancer patients are 65 years old or older, these older patients are severely underrepresented in breast cancer clinical trials. This study tested whether older patients were offered trials significantly less often than younger patients and whether older patients who were offered trials were more likely to refuse participation than younger patients. Patients and Methods: In 10 Cancer and Leukemia Group B institutions, using a retrospective case-control design, breast cancer patients eligible for an open treatment trial were paired: less than 65 years old and ≥ 65 years old. Each of the 77 pairs were matched by disease stage and treating physician. Patients were interviewed as to their reasons for participating or refusing to participate in a trial. The treating physicians were also given questionnaires about their reasons for offering or not offering a trial. Results: Sixty-eight percent of younger stage II patients were offered a trial compared with 34% of the older patients (P = .0004). In multivariate analyses, disease stage and age remained highly significant in predicting trial offering (P = .0008), when controlling for physical functioning and comorbidity. Of those offered a trial, there was no significant difference in participation between younger (56%) and older (50%) patients (P = .67). Conclusion: In a multivariate analysis including comorbid conditions, age and stage were the only predictors of whether a patient was offered a trial. The greatest impediment to enrolling older women onto trials in the setting of this study was the physicians' perceptions about age and tolerance of toxicity.