A 43-year-old female with a past history of breast carcinoma presented with headaches and dizziness concerning for metastatic disease. An MRI showed a large non-enhancing right cerebellar mass. Microscopic examination of the subsequent biopsy showed fragments of cerebellar parenchyma with disorganised collections of atypical ganglion cells of variable size consistent with dysplastic gangliocytoma/Lhermitte-Duclos disease (LDD). LDD is a rare, slow growing benign lesion of the cerebellum. The histogenesis remains uncertain as it is not clear whether it represents a hamartomatous or neoplastic process. Grossly there is enlargement and coarsening of the cerebellar folia. Histologically there is relative preservation of architecture with diffuse enlargement of the molecular and internal granular layers which contain atypical ganglion cells of various size and reduced numbers of purkinje cells. Adult onset of LLD is pathognomonic of Cowden syndrome a hamartoma/cancer syndrome associated with a germline loss of function mutation of the PTEN gene resulting in an increased risk of breast, thyroid, endometrial, colorectal and renal cell carcinoma. While LDD is a slow growing lesion without reported malignant potential, the association with Cowden syndrome and subsequent increased risk of epithelial malignancy warrants referral for genetic counselling.