Background: The presence of circulating tumor cells (CTCs) has been shown to be of prognostic relevance for patients with early and advanced breast cancer (BC). The usefulness of CTC assessments depends on accurate cell counts and corresponding analysis of molecular targets. The aim of this sub study was to assess the feasibility of a novel, integrated CTC platform for automated cellular protein and nucleic acid analysis in a prospective multi-center study. Methods: The German 4EVER study included patients with postmenopausal, metastastic, ER positive, HER2 negative BC, who progressed after therapy with a non-steroidal aromatase inhibitor and were treated with exemestane and the mTOR inhibitor everolimus. Baseline blood samples (TransFix vaccum tubes, Cytomark, Caltag Medsystems Ltd.) were used for CTC analysis and processed on the modified Versant kPCR Sample Prep system using 8µm pore size Whatman Nuclepore track-etched membranes (GE Healthcare Piscataway, NJ). After CTC capture, immunostaining was performed for Cytokeratin 8/18/19 and CD45. CTCs were detected by image analytics after fluorescence scanning microscopy using a dedicated software solution implemented by Siemens. The data is summarized and correlated with baseline clinical characteristics. Results: CTC counts and clinical data were available for 111 blood samples (91.7%). CTCs were found in 75 patients (67,6%), 13 patients having 1 CTC, 38 having 2-9 CTCs and 24 patients having 10 or more CTCs. In an exploratory analysis the presence of CTCs was correlated with baseline patient and tumor characteristics. There were no associations with primary TNM stage, hormone receptor status or tumor type. Conclusions: CTC assessment with this novel filtration based method was feasible in a multi-center study setting. The CTC positivity rate was within the expected range. The follow-up of this study will give first insights, how the CTC measures of this platform can be used as a prognostic tool. As this CTC assessment platform was developed to perform additional automated cellular protein and nucleic acid analysis, the usefulness might derive from these analytic tools as well. Clinical trial information: NCT01626222.