AIM: To describe the proportion of women in Tayside, Scotland diagnosed with early breast cancer who would have been eligible for influential adjuvant breast cancer trials. METHODS: Phase III trials of adjuvant treatment for breast cancer referenced in five national guidelines were shortlisted by breast cancer specialists to identify the twelve considered most influential. Eligibility criteria were extracted from protocols and applied to a 16-year cohort of women who had received a diagnosis of breast cancer and the proportion meeting trial criteria calculated. The criteria used clinically in Tayside to make decisions about use of the trial treatments were also applied to the cohort. Finally, the proportion of women receiving adjuvant endocrine therapy as part of their care and who would have been eligible for the trial evaluating that therapy was calculated. RESULTS: Of the cohort's 4811 women, 3535 (73%) were eligible for at least one trial but eligibility for an individual trial rarely exceeded 45%. There were substantial differences between the proportion of women meeting trial eligibility criteria and the proportion considered clinically eligible for the same treatment. The proportion of women receiving an endocrine therapy as part of their care who would also have been eligible for the trial evaluating that treatment ranged from 17% to 56%. CONCLUSION: Clinical eligibility criteria may be at variance with trial criteria. For adjuvant endocrine therapy, a substantial proportion of women who would have been ineligible for a trial nevertheless received the trial treatment as part of their care.