A 64-year-old man had progressive unsteadiness over several years, with tingling in his feet. He was a longstanding bodybuilding enthusiast. Clinical assessment and neurophysiology confirmed a cerebellar ataxia and axonal peripheral neuropathy. His serum mercury concentration was significantly raised. We diagnosed chronic mercury toxicity secondary to excessive tuna consumption. We advised him to stop eating tuna and prescribed dimercaptosuccinic acid, after which his serum mercury concentrations subsequently fell. This case report highlights the importance of considering dietary and nutritional causes of neurological disease. We also discuss the mechanisms, diagnosis and treatment of mercury toxicity.