Abstract Purpose The addition of trastuzumab to adjuvant chemotherapy has improved the outcome of human epidermal growth-factor receptor 2 (HER2)-positive breast cancer. Uncertainty remains about the optimal timing of trastuzumab treatment. Therefore, we compared long-term outcome after concurrent versus sequential treatment, in a population-based setting, using data from the nationwide Netherlands Cancer Registry. Methods We identified 1843 women diagnosed in The Netherlands from January 1st 2005 until January 1st 2008 with primary, HER2-positive, T_1-4N_anyM₀ breast cancer who received adjuvant chemotherapy and trastuzumab. Kaplan–Meier survival estimates and Cox regression were used to compare recurrence-free survival (RFS) and overall survival (OS) between women who received trastuzumab concurrently with versus sequentially after chemotherapy. Hazard ratios (HR) were adjusted for age, year of diagnosis, grade, pathological T-stage, number of positive lymph nodes, ER-status, PR-status, socio-economic status, radiotherapy, hormonal therapy, ovarian ablation, and type of chemotherapy. Results After a median follow-up of 8.2 years, RFS events had occurred in 224 out of 1235 (18.1%) concurrently treated women and 129 out of 608 (21.2%) sequentially treated women (adjusted-HR 0.91; 95% confidence interval (CI) 0.67–1.24; P = 0.580). Deaths occurred in 182/1235 (14.7%) concurrently treated women and 104/608 (17.1%) sequentially treated women (adjusted-HR 0.92; 95% CI 0.65–1.29; P = 0.635). Conclusions The results of this population-based study are consistent with earlier randomized trials, demonstrating a non-significant difference in outcome for concurrently treated women compared to those who were treated sequentially, suggesting both options are justified.