Recent imaging studies have implicated the anterior cingulate cortex (ACC) in various cognitive functions, including attention. However, until now, there was no evidence for changes in neuronal activity of individual ACC neurons during performance of tasks that require attention and effortful thought. We hypothesized these neurons must exist in the human ACC. In this study, we present electrophysiological data from microelectrode single neuron recordings in the human ACC of neuronal modulation during attention-demanding tasks in 19% of 36 neurons tested. These findings provide the first direct evidence of an influence of a cognitive state on the spontaneous neuronal activity of human ACC neurons.