Trastuzumab in conjunction with adjuvant chemotherapy markedly improves outcome. In the Netherlands, a national guideline was released in September 2005 stating that trastuzumab should be given in conjunction with adjuvant chemotherapy in women with HER2-positive breast cancer. Aim of this study was to identify the number of women with HER2-positive breast cancer and to evaluate the level of implementation of adjuvant trastuzumab in clinical practice nationwide. Women diagnosed with primary breast cancer between September 2005 and January 2007 were selected from the Netherlands Cancer Registry (NCR). HER2 status, adjuvant treatment and reasons to withhold trastuzumab were registered. 14,934 Breast cancer patients were diagnosed in this period of whom 1,928 (13%) had a HER2-positive tumour. Of all HER2-positive women receiving adjuvant chemotherapy, 66 (6%) did not receive trastuzumab. This percentage decreased from 10% at the time of introduction of the guideline to 4% in the study period September 2005-December 2006. Most common reasons to withhold trastuzumab were cardiovascular disease (29%) and patient refusal (21%). Of all HER2-positive patients who received adjuvant chemotherapy, 94% received trastuzumab. The implementation of trastuzumab in clinical practice was realized within 8 months after introduction of the new guideline.