Background Comorbid attention-deficit/hyperactivity disorder (ADHD) is common in bipolar disorder and associated with worse outcomes. Cognitive testing might be a tool to identify this group. Here we compare the neuropsychological profiles of bipolar disorder patients with (BD + cADHD) and without (BD − cADHD) childhood attention-deficit hyperactivity disorder. Methods Adult patients with BD  −  cADHD (n = 66), BD + cADHD (n = 32), and healthy controls (n = 112) were tested using a comprehensive battery of neuropsychological tests. Patients underwent rigorous diagnostic assessments for bipolar disorder and ADHD, as well as a parental interview to establish childhood ADHD. Results The neuropsychological profiles of the groups were similar, except that the BD + cADHD group performed significantly worse on working memory. Working memory did not differ between those in the BD + cADHD group who only had a history of childhood ADHD and those that still met criteria for ADHD in adulthood. Conclusions Cognitive testing had limited power to differentiate between bipolar disorder adults with and without childhood ADHD. The BD + cADHD subgroup cannot explain the significant cognitive heterogeneity seen in bipolar disorder patients. Electronic supplementary material The online version of this article (10.1186/s40345-019-0149-9) contains supplementary material, which is available to authorized users.