This study was to determine if the use of regional radiotherapy (RT) changed in British Columbia after publication of new randomized trial data in 1997. Women with pathologic T1-3N1, nonmetastatic breast cancer treated with a mastectomy or breast-conserving surgery (BCS) were included. The use of regional RT was compared in two cohorts: cohort 1, July 1, 1995-June 30, 1997 (n = 834); and cohort 2, July 1, 1998-June 30, 2000 (n = 1072). All p-values were two-sided. Adjuvant systemic therapy was given to 96% and 95% of women in cohorts 1 and 2, respectively. Forty-five percent of cohort 1 and 48% of cohort 2 had BCS. Regional RT was received by 44% of cohort 1 and 66% of cohort 2 (p < 0.001). Eighty-eight percent and 90% of women with four or more positive nodes in cohorts 1 and 2 received regional RT, respectively. For women in cohorts 1 and 2 with one to three positive nodes, regional RT use increased from 32% to 54% after mastectomy, and from 23% to 59% after BCS, respectively (p < 0.001 for both). Publication of randomized trials and a coordinated guideline implementation process in British Columbia was associated with a significant increase in the use of regional RT in women with one to three positive nodes.