Euthymic patients with bipolar disorder have been reported to show persistent deficits in sustained attention. However, the sustained attention task which was used also placed demands on working memory. Bipolar disorder patients in the euthymic state were therefore compared with healthy controls on two measures of sustained attention with and without a working memory component. Signal detection methodology was applied to the results. Euthymic patients with bipolar disorder were particularly impaired at detecting targets in the sustained attention task without a working memory component. This deficit was still apparent in a sub-group of patients who were not currently receiving lithium medication. By contrast, performance in the sustained attention task involving working memory task was not significantly different in the two groups. Sustained attention deficits apparent during the euthymic period of bipolar disorder cannot be explained in terms of working memory impairment and represents a reduced inherent capacity rather than a change in response bias. Deficits in sustaining attention may help explain the difficulties in psychological and occupational functioning in bipolar disorder patients during remission.