Primary central nervous system (CNS) lymphoma is an aggressive malignancy which constitutes one of the acquired immunodeficiency syndrome -defining illnesses. Early diagnosis and timely management can increase the chances of cure. Although many times the diagnosis is straightforward, we present a case of primary CNS lymphoma in a human immunodeficiency virus--positive individual which posed as a major diagnostic dilemma with initially normal imaging findings. A 42-year-old male presented with unremitting fever and a perianal ulcer for 3 months. A battery of diagnostic tests were negative, including a positron emission tomography-computed tomography scan and a magnetic resonance imaging brain. With unresolving symptoms and a high index of suspicion as he developed dizziness and loss of balance, the same were repeated which confirmed a space-occupying lesion in the cerebellum. Although treatment was instituted, the patient did not recover and died in the 4th month of treatment.