Bevacizumab (Avastin, Genetech/Roche) is an anti-angiogenic drug approved for treating patients with malignant gliomas that reduces edema and mass effect, but has been suggested to promote multifocal tumor spread within the brain. Patients with systemic malignancies are also treated with bevacizumab, but there is limited information regarding effects of the drug on the neuroimaging or neuropathological features of metastatic CNS disease. We report 2 patients with non-small cell lung carcinomas who had received bevacizumab for their systemic cancers and then developed cognitive deficits consistent with white matter dementia. Diagnosis of leptomeningeal carcinomatosis (LC) was confounded and delayed by the finding of atypical neuroimaging features, including minimal to absent leptomeningeal enhancement and unusual perivascular and punctate hemorrhagic lesions and multifocal subgyral signal abnormalities suspicious for vasculitis or small vessel vasculopathy. Neuropathological assessment confirmed LC but, in the autopsy case also disclosed extraordinary perivascular spread of individual metastatic tumor cells to the depth of capillaries. The pattern was reminiscent of vascular "cooption" by tumor seen in experimental animals in preclinical trials of bevacizumab. Small infarctions were associated with perivascular tumor and vasculopathy, unusual features of LC in patients who do not receive bevacizumab. In the biopsied patient, multiple perivascular tumor nodules were identified in superficial cortex. In these two patients, bevacizumab appeared to alter neuroimaging characteristics of LC, confounded diagnosis and possibly also influenced the pattern of tumor spread of LC. More cases will need to be studied to confirm this latter finding.