Definitive ante-mortem diagnosis of intracranial lesions is not without inherent technical and diagnostic challenges. The introduction of new technology, such as MRI guided, minimally invasive brain biopsy, has improved diagnostic ability; however, even with lesion biopsy, discerning between etiologies is not always guaranteed. This report describes the case of a dog with diffuse central nervous system disease. Magnetic Resonance Imaging detected two contrast enhancing lesions, one in the cerebellum, and on in the forebrain. A BrainSight stereotactic ante-mortem biopsy was obtained from the cerebellar lesion, revealing an infiltrate of histiocytic cells, mixed with fewer lymphocytes and plasma cells, consistent either was granulomatous inflammation or a histiocytic neoplasm. The patient continued to decline rapidly, and was humanely euthanized. On necropsy, the lesions were examined more closely, identifying a neoplastic process, and making a definitive diagnosis of malignant histiocytosis.