NVP-AUY922, a potent heat shock protein (HSP) 90 inhibitor, downregulates the expression of many oncogenic proteins, including the human epidermal growth factor receptor-2 (HER2). Because HER2 downregulation is a potential biomarker for early response to HSP90-targeted therapies, we used the (89)Zr-labelled HER2 antibody trastuzumab to quantify the alterations in HER2 expression after NVP-AUY922 treatment with HER2 positron emission tomography (PET) imaging. The HER2 overexpressing human SKOV-3 ovarian tumour cell line was used for in vitro experiments and as xenograft model in nude athymic mice. In vitro HER2 membrane expression was assessed by flow cytometry and a radio-immuno assay with (89)Zr-trastuzumab. For in vivo evaluation, mice received 50mg/kg NVP-AUY922 intraperitoneally every other day. (89)Zr-trastuzumab was injected intravenously 6d before NVP-AUY922 treatment and after 3 NVP-AUY922 doses. MicroPET imaging was performed at 24, 72 and 144h post tracer injection followed by ex-vivo biodistribution and immunohistochemical staining. After 24h NVP-AUY922 treatment HER2 membrane expression showed profound reduction with flow cytometry (80%) and radio-immuno assay (75%). PET tumour quantification, showed a mean reduction of 41% (p=0.0001) in (89)Zr-trastuzumab uptake at 144h post tracer injection after NVP-AUY922 treatment. PET results were confirmed by ex-vivo (89)Zr-trastuzumab biodistribution and HER2 immunohistochemical staining. NVP-AUY922 effectively downregulates HER2, which can be monitored and quantified in vivo non-invasively with (89)Zr-trastuzumab PET. This technique is currently under clinical evaluation and might serve as an early biomarker for HSP90 inhibition in HER2 positive metastatic breast cancer patients.