A 53-year-old man presented with malignant lymphoma manifesting as disturbances of walking and standing. Magnetic resonance (MR) imaging showed multifocal bifrontal lesions which were enhanced by gadolinium-diethylenetriaminepenta-acetic acid. Positron emission tomography (PET) with [18F]fluorodeoxyglucose showed high uptake of tracer in the lesion. The PET-MR coregistered image was used to determine the biopsy target. After right frontal craniotomy, a sterilized probe controlled by a neuronavigation system was directly passed into the tumor, and a guide tube was inserted along the same track. After dura opening, a small corticotomy was performed along the guide tube track and the tumor was biopsied. Histological examination revealed malignant lymphoma. The stereotactically inserted tube-guided brain biopsy was less invasive and provided an accurate diagnosis. The PET-MR coregistered image was helpful for determining the most active lesion of the brain tumor.