Cognitive impairment is often identified in individuals with bipolar disorder and is associated with their functional impairment. However, there is controversy surrounding potential classification methods for impairment in cognitive measures.To examine the proportion of cognitive measures indicating impairment of attention, processing speed, memory, visuoconstructional abilities, and executive functions in individuals with bipolar disorder type I (euthymic) and healthy controls, using a strict criterion for defining impairment.We gave 43 individuals with bipolar disorder type I and 17 healthy controls a comprehensive clinical and neuropsychological assessment. All scores were standardized using means and standard deviations according to age. Impaired performance in all cognitive measures was determined using a distribution-based threshold of z=±1645. The effects of the sociodemographic and clinical variables on cognitive performance were examined using multiple stepwise backward regression analyses.Clinically significant cognitive impairment was observed more frequently in the bipolar disorder group, compared to controls, on all measures. From participant factors, we found that level of education and number of manic episodes predicted variation in more cognitive measure scores.The use of population-based norms to standardize cognitive measures, and a strict criterion to define cognitive impairment, in individuals with bipolar disorder type 1 and healthy controls resulted in a prevalence of impairment in cognitive domains' frequencies of deficits that fell within the ranges previously reported in meta-analyses.Clinically introducing population norms and a stringent cognitive impairment criterion can facilitate more accurate measures of cognitive impairment in individuals with bipolar disorder.