Neurological patients were presented with a succession of 2-11 stimuli which they were required to count, reporting the number in the series when it finished. The stimuli were binaural clicks, or pulses on the right or on the left index finger. Regardless of stimulus modality or lateralization, patients with lesions involving the right frontal lobe were impaired when the presentation rate was 1/sec. There was no corresponding impairment when the presentation rate was increased to 7/sec. It is argued that at slow rates when the task was monotonous patients with right-frontal lesions were less able than others to sustain attention voluntarily.