Obsessive-compulsive disorder (OCD) and attention-deficit and hyperactivity disorder (ADHD) frequently coexist. To understand whether childhood ADHD can increase the risk of OCD in adulthood and whether it influences the phenomenology of OCD, we investigated the symptoms of ADHD during childhood in obsessive-compulsive adults who had never been diagnosed as ADHD. Adults with OCD (n = 83) were given the Wender Utah Rating Scale (WURS), Yale-Brown Obsessive Compulsive Scale (Y-BOCS), Barratt Impulsiveness Scale-11 (BIS-11), Hamilton Depression Rating Scale-17 (HDRS-17) and Beck Anxiety Inventory (BAI). The prevalence of childhood ADHD symptoms was 40.9 % (n = 34) and that of adult ADHD was 16.9 % (n = 14). Patients with childhood ADHD symptoms had an earlier onset of OCD, higher scores of the BAI and BIS-11. The scores of the Y-BOCS and HDRS-17 did not differ between those having and not having childhood ADHD symptoms. Childhood history of ADHD symptoms is common in adult OCD patients who have never been diagnosed as ADHD. Childhood ADHD symptoms are associated with an earlier age of OCD, more severe anxiety and higher impulsiveness. Even remitted ADHD may be a risk factor for OCD in later life.