['To investigate in a prospective study the prognostic value of serum HER2 extracellular domain (ECD) level in patients with primary breast cancer overexpressing HER2 treated with adjuvant chemotherapy and trastuzumab.', 'All the patients treated for primary breast cancer with chemotherapy and adjuvant trastuzumab from April 1, 2005 to December 31, 2006 at the Centre de Lutte Contre le Cancer de Haute Normandie were enrolled in this prospective study. HER2 ECD was measured in frozen serum by a commercial kit with a cut-off value of 15 ng/ml.', 'Sixty-five patients were enrolled. Seven patients (11%) had an elevated serum HER2 ECD level (mean=25.1 ng/ml, range 15.1-38.9 ng/ml). During follow-up, 13 patients (20%) developed metastases and seven patients (11%) died. Death was related to breast cancer metastases in six patients (9%). Out of the seven patient with elevated serum HER2 ECD level, five (71%) developed metastases and three (43%) died of metastases during follow-up. Multivariate analysis showed that elevated serum HER2 ECD level was the unique factor for both disease-free survival (p<0.0006) and overall survival (p=0.008) in this series.'] Does elevated HER2 extracellular domain level in primary breast cancer with HER2 overexpression predict early failure of adjuvant trastuzumab?