The porphyrias are a group of rare hereditary metabolic disorders where there is an excess formation and excretion of porphyrins or their precursors. Type IIA, acute intermittent porphyria (AIP), has an estimated prevalence of one to eight per 100,000 in the general population but is thought to have a higher prevalence in psychiatric patients. AIP can present with a variety of psychiatric symptoms, often misdiagnosed. Associated neuropathological changes including focal cerebral ischaemic lesions have been found. However, to our knowledge, no case of dementia and AIP has been described. We present the case of a 56 year old man with a five-year history of progressive cognitive decline, diagnosed with AIP at an advanced stage of dementia. Whether AIP contributed to the dementia or is a coincidental finding is unknown. However treatment of AIP in this case resulted in some improvement in the patient's cognitive state.