A 51 year old man with biopsy proven pulmonary sarcoidosis and skin test positive for tuberculosis presented with features of an amelanotic flat choroidal mass suggestive of choroiditis. The mass enlarged despite corticosteroids and anti-tuberculous medications. A thorough systemic evaluation for possible primary tumor metastatic to the choroid was negative. Further clinical evaluation and magnetic resonance imaging suggested a diffuse primary choroidal malignant melanoma with optic nerve invasion. The eye was enucleated and the mass proved histopathologically to be a mucin secreting adenocarcinoma of unknown origin despite a repeat systemic work-up. The patient died three months after the onset of symptoms and three weeks after enucleation with diffuse metastases from an unknown primary cancer. Magnetic resonance imaging (MRI) is usually helpful in the differentiation of uveal melanoma from uveal metastasis. In this case, however, it suggested the diagnosis of a diffuse choroidal melanoma. The reason for the atypical MRI findings will be discussed.