This study evaluated the impact of province-wide treatment guidelines on consistency of adjuvant therapy for node-negative breast cancer. A retrospective population-based cohort study was conducted in the Canadian provinces of British Columbia, which has province-wide guidelines, and Ontario, which does not. All eligible 1991 incident cases of node-negative breast cancer in British Columbia (n = 942) and a similar number of randomly selected 1991 incident cases in Ontario (n = 938) were reviewed. Consistency of adjuvant therapy received was evaluated by stratifying cases into discrete diagnostic groups using several grouping systems, and by then comparing the distribution of treatments received within each diagnostic group in the two provinces. Recursive partitioning was also performed. We observed that patterns of pathology reporting were consistent with awareness of the factors used in the British Columbia guidelines to define indications for adjuvant therapy. Consistency of care was greater in British Columbia than in Ontario by all diagnostic grouping systems and by recursive partitioning (P < 0.001), and the observed patterns in British Columbia corresponded to the British Columbia guidelines. We conclude that population-based treatment guidelines can play a role in promoting consistent patterns of adjuvant therapy for women with node-negative breast cancer.