Purpose: VEGF is one of the most important angiogenesis-stimulating cytokines and has been previously shown to be overexpressed in several solid cancers. The aim of the present study was to assess the clinical relevance of serum VEGF (sVEGF) in a large cohort of metastatic breast cancer patients and to explore the relationship between sVEGF and other blood-based biomarkers. Methods: Two hundred fifty-three patients with metastatic breast cancer were enrolled in this prospective, multicentre study. Blood samples were collected before start of first-line or later-line treatment. sVEGF was quantified by a commercially available ELISA. Circulating tumor cells (CTCs) were detected using CellSearch and other biomarkers (EGFR, HER2, RAS p21, TIMP1, CAIX) by ELISA. Results: Levels of sVEGF were determined in all patients, with a median concentration of 231 pg/ml. After a median follow-up of 19 months, median overall survival (OS) was 10.2 months in patients with sVEGF levels above the upper quartile (i.e. 367 pg/ml), while median OS has not been reached in patients with sVEGF < 367 pg/ml (p < 0.001). Median progression-free survival (PFS) was 4.8 months for patients with sVEGF ≥ 367 pg/ml versus 9.1 months with sVEGF levels < 367 pg/ml (p < 0.001). Patients with sVEGF levels ≥ 367 pg/ml and ≥ 5 CTCs had the shortest OS, while those with sVEGF < 367 pg/ml and non-elevated CTCs had the longest OS. CTCs, grading, line of therapy and RAS p21 were independent predictors of OS. sVEGF, line of therapy and CTCs were independent predictors of PFS in the multivariate analysis. Conclusions: Metastatic breast cancer patients with elevated levels of sVEGF have significantly worse clinical outcome. This finding supports the biological role of VEGF in breast cancer. Trial registration: Current Controlled Trials ISRCTN59722891 (DETECT).