PURPOSE The aim of this study was to evaluate the prognostic quality of adjacent vessel sign (AVS) in malignant breast tumors by comparing it with classical prognostic pathological biomarkers and magnetic resonance imaging (MRI) findings. METHODS A total of 124 patients with 133 malignant lesions were included. All the imaging was performed on a 1.5T Avanto scanner and the images were interpreted according to BI-RADS-MR® (fifth ed.) atlas. Maximum intensity projection (MIP) images were constructed from subtracted post-con trast images and were used to investigate AVS. Histopathological results and MRI findings were compared with AVS. RESULTS Interobserver agreement about AVS status was substantial (κ=0.64). AVS positive lesions were significantly bigger in size (P < .001, AVS negative: median 12 mm, AVS positive: median 31 mm). AVS was significantly associated with increased Ki-67 index and axillary lymph node metasta sis (P=.009 and P=.019, respectively). Between AVS and lymphovascular invasion (LVI), there was a trend toward positive relationship (P=.076). MRI findings of T2 hypointensity, peritu moral edema, irregular shape, non-homogeneous contrast enhancement, rapid early contrast enhancement, and skin infiltration showed significant positive relation with AVS (P < .001, P < .001, P < .001, P=.02, P=.021, and P=.021, respectively). AVS is found to be associated with increased Ki-67 index, axillary lymph node metastasis, and some MRI findings that point to malignancy or poor prognosis. CONCLUSION AVS indicates poor prognosis since it is related to axillary lymph node metastasis, increased Ki-67 index, LVI, peritumoral edema, rapid early contrast enhancement, increased background enhancement, skin extension, T2 hypointensity, non-homogeneous contrast enhancement, irregular lesion shape, and larger tumor size. AVS is an easy to use sign that shows substantial interobserver agreement.