Impairments of attention are common among people with major affective disorders, yet the influence of effortful task demands on attentional performance in unipolar and bipolar illness has been little studied. The authors compared psychiatric inpatients with primary diagnoses of unipolar or bipolar affective disorder (n=27) and age-matched normal control subjects (n=20) on a battery of eight neuropsychological tasks designed to measure different attentional functions. There were low-effort and high-effort versions of each task. Significant group differences were consistently observed on tasks demanding sustained and focused attention, but not on tasks requiring visual selective attention. Although affective disorder patients showed impairments on most tasks regardless of level of task effort, group differences were greatest on high-effort conditions. Results indicate that patients with major affective disorders show significant attentional impairments on most measures of effortful attention, and the magnitude of these impairments increases as the effortful demands of the task increase.