Gliomatosis confined to the cerebellum is most unusual. We report such a case in a 20-month-old male who presented with unsteadiness. Magnetic resonance imaging revealed a diffuse area of abnormal signal intensity within both cerebellar hemispheres, which did not enhance after contrast administration. The patient underwent a biopsy, which revealed a diffuse glioma infiltrating the cerebellum. Overall, the tumor cells had oligodendroglioma-like features and exhibited only focal vimentin immunoreactivity. They were negative for glial fibrillary acidic protein, synaptophysin, βIII-tubulin, and neurofilament protein. Immunofluorescence, performed on primary biopsy explants maintained in cell culture without exposure to growth factors or differentiation-promoting agents, revealed widespread nestin immunoreactivity and immunolabeling of occasional cells with antibodies to platelet-derived growth factor-α and O1/O4, markers of oligodendrocyte precursor-cells and immature oligodendrocytes, respectively. Fluorescent in situ hybridization performed on explants, touch preparations, and paraffin sections failed to reveal loss of heterozygosity for either 1p36 or 19q13. The patient was treated with temozolomide and remains stable, albeit with residual quiescent tumor, more than 3 years after surgery. This report calls attention to an unusual presentation of gliomatosis confined to the cerebellum of a toddler and addresses salient aspects of clinical and radiological differential diagnosis, as well as therapeutic challenges encountered.