OBJECTIVE: According to data from animal models, complement activation plays a major role in the brain injury after acute ischemic stroke. Scarce findings are, however, available on the detection of complement activation products in stroke patients. METHODS: We have measured plasma levels of the five complement activation products (C1rC1sC1inh, C4d, C3a, C5a and SC5b-9) in samples of 26 patients with ischemic stroke upon admission. Twenty-six patients with severe carotid atherosclerosis served as patient controls. RESULTS: Levels of two activation products (SC5b-9 and C4d)) were significantly elevated in the plasma of stroke patients, SC5b-9 levels, exhibited significant positive correlation with the clinical severity of stroke, the severity of neurological deficit, as well as with the level of functional disability. CONCLUSION: These findings suggest that complement activation plays an active role in the development of brain infarct. The measurement of complement activation products might help to determine the clinical prognosis after acute ischemic stroke. Furthermore, there is potential usefulness of complement modulating therapy in ischemic stroke.