•What is the primary question addressed by this study?To confirm that Attention Deficit Hyperactivity Disorder (ADHD) is associated with increased risk for developing dementia and Lewy body disease(LBD).•What is the main finding of this study?ADHD is independently associated with an increased risk of LBD, dementia, and non-amnestic-Mild Cognitive Impairment.•What is the meaning of the finding?As adults with ADHD appear to be at higher risk for cognitive decline and LBD, further studies should be conducted to elucidate this association and develop preventive interventions. Past reports have suggested that attention-deficit/hyperactivity disorder (ADHD) may be a risk factor for Lewy body disease (LBD). To confirm this relationship, we conducted the present study. A prospective observational cohort study with a follow-up to 15 years. The subjects were recruited from cognitive neurology clinics, where they attended for a cognitive complaint or health check-up. Two groups of subjects: ADHD adults and healthy subjects. The risk of dementia and LBD was estimated with Kaplan-Meier analysis comparing for the presence or absence of ADHD with the log-rank test. Predictors of conversion were assessed through separate univariate and multivariate Cox regression analyses, adjusting for several variables. The baseline sample consisted of 161 subjects with ADHD and 109 without ADHD. At the end of the follow-up, 31 subjects developed dementia, 27 cases in the ADHD group and 4 in comparison group. Dementia with Lewy bodies (DLB) was the most frequent type (N:20) of which 19 corresponded to the ADHD group. The incidence of non-amnestic-MCI in the ADHD group was higher representing 67.1 % of these subjects (N:108), and 17.4% (N:19) of healthy cases. The hazard ratios for dementia and LBD in the multivariate adjusted model were 3.33 (95% CI 1.0915 to 10.1699) and 54.54 (95% CI 7.4849 to 397.5028), respectively in the ADHD group. This study showed that adult ADHD is independently associated with an increased risk of LBD, dementia, and na-MCI. Future studies should clarify this relationship to develop preventive measures for these patients.