A 37 year old man presented with progressive dysarthria for 2 weeks. A week later he developed ataxia and bilateral cerebellar signs including intention tremors, dysmetria and dysdiadokokinesia. During evaluation for aetiology of cerebellar dysarthria, MRI brain revealed asymmetric altered signal intensities in bilateral cerebellar hemispheres and right side of pons suggesting demyelinating lesions. ELISA for Human Immune Deficiency virus-1 was positive. We kept a presumptive diagnosis of Progressive Multifocal Leukoencephalopathy (PML) on the basis of clinico-radiological picture. PML is an under investigated and under diagnosed CNS infection seen in HIV patients with advanced disease. We present an unusual case report where isolated cerebellar involvement occurred as the first AIDS defining event in the absence of appreciable immunodeficiency in a patient with previously undiagnosed HIV infection.