Interleukin-15 is a novel proinflammatory cytokine. It is produced by activated blood monocytes, macrophages, and glial cells. The objective of our study was to assess the role of interleukin-15 as a marker of increased proinflammatory activity in patients with Alzheimer's disease and frontotemporal dementia. We measured cerebrospinal fluid interleukin-15 levels in 17 patients with Alzheimer's disease and 7 patients with frontotemporal dementia in comparison with 17 patients with amyotrophic lateral sclerosis and 15 patients with Parkinson's disease. Patients with Alzheimer's disease and frontotemporal dementia had significantly higher cerebrospinal fluid interleukin-15 levels compared with patients with noninflammatory neurological diseases (P < .05 and P < .01, respectively). In Alzheimer's disease, a significant positive correlation was noted between interleukin-15 levels and age of onset (R = .48, P = .05). Our findings suggest that interleukin-15 may be implicated in the pathophysiology of Alzheimer's disease and frontotemporal dementia.