High levels of antibodies to glutamic acid decarboxylase (anti GAD-Ab) are an uncommon association in neurological conditions. We describe a patient who initially presented with cerebellar ataxia. Four years later she was diagnosed with Insulin dependent diabetes mellitus (IDDM). One year after this she presented with complex drug resistant partial seizures. High anti GAD-Ab levels have been recorded in paired samples of her CSF and serum. Intra thecal synthesis (IS) of anti GAD—Ab and IS of IgG was also demonstrated. Since up to 80% of patients with IDDM can be positive for anti GAD-Ab, it was important that we could demonstrate IS of GAD-Ab to confirm that the GAD autoimmunity is related and possibly contributes to the neurological syndrome rather than to the concomitant IDDM. As anti GAD-Ab cannot cross the blood brain barrier IS of anti GAD-Ab can explain CNS symptoms such as epilepsy where anti GAD-Ab activity may be actively impairing the GABA synaptic transmissions—GABA being an important inhibitory neurotransmitter We review the patients' clinical presentation and discuss her management.