Levels of S100beta, a calcium-binding protein found in astrocytes, were measured using a sandwich ELISA in the cerebrospinal fluid (CSF) of patients with frontotemporal dementia and Alzheimer's disease and compared with controls. Mean CSF S100beta concentrations were significantly raised in patients with frontotemporal dementia when compared with healthy controls (0.49 +/- 0.28 vs. 0.22 +/- 0.08 ng/ml, P < 0.001). There was no correlation between age at disease onset, disease severity or length of illness. The increased concentration of CSF S100beta seen in frontotemporal dementia may reflect the marked astrocytosis seen in this condition.