Bipolar Affective Disorder (BPAD) is a common psychiatric illness with evidence supporting a high degree of genetic vulnerability. Current research is continuing to focus towards discovering possible genetic markers for the disorder, which in turn, may help in the eradication of the neurobiological mechanisms in the future. Essentially, neurocognitive endophenotypes may prove to be candidate markers in BPAD. 15 bipolar affective type 1 patients and 25 of their first-degree relatives were recruited from the St. Patrick's Hospital Bipolar Disorder Project in Dublin, Ireland. Global intelligence was measured in all individuals using the Wechsler Adult Intelligence Scale - Revised (WAIS-R). Executive function was assessed by using two psychometric tests. The tests used were the Wisconsin Card Sorting Test (WCST) and the Hayling Sentence Completion Task (HSCT). Normative data from the project database was used as controls. Relatives outperformed both controls and patients in the WCST. Controls were quicker than either relatives or patients in the HSCT, and patients had significantly more errors. Neurobiological correlates of the tests of executive function are discussed. Dorsal prefrontal function was found to be superior in relatives. Tests examining ventral prefrontal function showed dysfunction in the patient group while suggesting a more subtle dysfunction in the relative group. Ventral prefrontal cortical function may have potential as an endophenotypic marker for BPAD.