Introduction:  Previous reports have shown that in Dementia with Lewy body (DLB) and attention‐deficit and hyperactivity disorder (ADHD) a hypodopaminergic and noradrenergic substrate seems to play a central role in developing the diseases. We investigated the hypothesis that attention deficit may precede DLB expressed as adult ADHD symptoms long before the clinical onset of dementia.Methods:  Patients with DLB, Alzheimer disease type (ADT) and controls were recruited from the membership of the Italian Hospital Medical Care Program in Argentina from 2000 to 2005. The DSM‐IV criteria adapted for the identification of adult patients with ADHD and validated to Spanish Wender Utah Rating Scale were used to identify individuals with preceding ADHD symptoms during their adult life. Analysis of categorical variables was carried out using chi‐square. Mann–Whitney test was used for continuous variables. Statistical significance was P < 0.05.Results:  A total of 109 patients with DLB and 251 patients with ADT were matched by age, sex and year of education with 149 controls. The frequency of preceding ADHD symptoms in DLB cases was 47.8% in ADT 15.2% and 15.1% in the control group. The prevalence of ADHD symptoms in DLB cases was significantly higher compared with the control group (P ≤ 0.001, OR 5.1 95%CI 2.7–9.6) and also higher when compared with ADT (P ≤ 0.001, OR 4.9, 95%CI 2.8–8.4).Conclusion:  We found a higher risk of DLB in patients with preceding adult ADHD symptoms. To date, there is no clear explanation for the association found; however, further investigation will widen our understanding about both disorders.