['Serum HER2 (S-HER2) was approved in 2003 by the US Food and Drug Administration (FDA) for monitoring trastuzumab treatment in tissue HER2 positive breast cancer patients. Information of the value of S-HER2 is scarce. We hypothesised that S-HER2 would reflect the clinical effect of trastuzumab.', 'We followed 48 patients eligible for trastuzumab treatment for up to 6 years or until death. S-HER2 was measured on an ADVIA Centaur System and S-Trastuzumab was measured by an in-house developed fluorescent enzyme immunoassay system on the ImmunoCap 100.', 'A decrease in S-HER2 of ≥ 20% was correlated to no progression in the disease in 20 out of 21 clinical courses (p<0.0001). An increase in S-HER2 of ≥ 20% was correlated to progression in the disease in 40 out of 44 clinical courses (p<0.0001). Patients with no recurrence after trastuzumab treatment (n=18) had a median S-HER2 concentration of 10.5 μg/L, whereas patients alive with recurrence (n=13) had a median S-HER2 of 20.1 μg/L (p=0.002). Patients who died prompted by recurrence (n=17) had a median S-HER2 of 232.4 μg/L at latest measurement before death (p=<0.0001) compared to patients without recurrence. In two patients with S-HER2 values above 1000 μg/L the concentrations of S-trastuzumab were measured below the target trough concentration in serum of 10 mg/L.'] Does serum HER-2 predict response and resistance to trastuzumab treatment in breast cancer?