We present the case of an 81 year old man, admitted with ataxia and hyponatraemia. Biochemical analysis was suggestive of Syndrome of Inappropriate ADH (SiADH) which was initially attributed to a recent respiratory infection or treatment with omeprazole. Neurological symptoms worsened despite normalisation of sodium levels and further examination revealed generalised areflexia; subsequent investigation by the Regional Neurology team led to a diagnosis of Guillan Barre syndrome, and symptoms improved following treatment with Intravenous Immunoglobulin and prolonged in-patient rehabilitation. Guillan Barre syndrome is a recognised cause of SiADH and this case highlights the importance of considering this diagnosis along with full neurological examination when patients with hyponatraemia present with neurological symptoms.