Objective. Cognitive dysfunction in bipolar disorder (BD) is well established in the literature. The neurocognitive deficits have been considered to be endophenotypic markers of BD, and studies have examined whether neurocognitive deficits exist in first-degree relatives of individuals with BD I. We hypothesized that performance in tests of neurocognitive function would be impaired in euthymic BD I patients and their unaffected first-degree relatives compared to that of healthy controls. Methods. We compared the performance of bipolar patients, their first-degree relatives, and healthy controls in a battery of neurocognitive tests to reveal possible endophenotypes of BD. A diagnostic interview and neuropsychological test battery were administered to 30 BD I patients, 55 of their unaffected first-degree relatives and 32 healthy controls. Results. The patients and their first-degree relatives were significantly impaired in executive function assessed using the Wisconsin Card Sorting Test (WCST) and Trail Making Test-B (TMT-B) relative to the controls (WCST; perseverative errors : p < 0.0005, categories completed : p = 0.002, TMT-B; p = 0.002). There were no significant differences between the groups in terms of attention, psychomotor speed, verbal memory, or learning. Conclusion. Our study suggests that the deficits in executive function may be endophenotypic markers of genetic vulnerability to BD I.