The case of a 49-years old woman with severe presenile dementia caused by primary hyperparathyroidism is presented. Parathyroidectomy proved to have a beneficial influence both on the clinical picture and on brain-SPECT. Psychiatric symptoms are seen in four to 73% of patients with primary hyperparathyroidism. Dementia is rarely associated with primary hyperparathyroidism. It is hypothesised that parathyroid hormone causes neuropsychiatric symptoms via an increase of intracerebral calciumlevels and an influence on different neurotransmittorsystems. The presence of primary hyperparathyroidism should be considered in each patient presenting with dementia.