Phospholipase A2 (E.C.22.214.171.124., PLA2) is a key enzyme responsible for membrane phospholipid turnover. There are some findings that PLA2 participates in central nervous system pathology (e.g. multiple sclerosis, epilepsy and Alzheimer's disease). Our purpose was to estimate PLA2 activity in patients with Alzheimer’s disease, Parkinson’s disease and vascular dementia, using human platelets as a peripheral model of neuron. Material and Methods: The examined group consisted of 15 patients with Alzheimer’s disease, 13 with vascular dementia and 10 with Parkinson’s disease. The control group consisted of 15 healthy people. Human platelets were sonificated, than PLA2 activity was measured performed according to Jelsema as well as Strosznajder and Strosznajder with slight modifications. The Student t-test was used for statistical evaluation of the data. Results: PLA2 activity in blood platelets of patients with Alzheimer's disease was 1.05 ± 0.44 (nmol/mg/min, mean ± SD), with Parkinson’s disease 0.67 ± 0.3, and in platelets of the patients with vascular dementia 0.78 ± 0.39. The activity in platelets of the control was 0.39 ± 0.13. Conclusion: Activity of platelet PLA2 is increased in patients with Alzheimer's dementia, Parkinson's disease and vascular dementia.