Patterns of functional cortical activation were studied by means of regional cerebral blood flow measurements, performed during rest and during a word fluency task in normal subjects (n = 22), in patients with Alzheimer's disease (n = 17), and in patients with frontotemporal dementia (n = 15). Although all groups showed a significant activation of the Broca's area during word production, the activation of the dorsolateral prefrontal cortex was clearly subnormal in both dementia groups. The frontal dysfunction was not explained by number of words produced, illness duration, or age. Thus, the results demonstrate that the word fluency task is a sensitive measure of frontal lobe function, and its incorporation in imaging studies may facilitate the detection of subtle functional impairment of the frontal lobes in organic dementia.