We explored the possibility of diagnostic confusion between hypersomnias of central origin (narcolepsy and idiopathic hypersomnia, IH) and the adult form of attention-deficit/hyperactivity disorder (ADHD). We included 67 patients with narcolepsy, 7 with IH and 61 with ADHD. All patients completed the Epworth Sleepiness Scale and the ADHD Rating Scale. We found that 18.9% of the hypersomnia patients fulfilled the self-reported criteria for ADHD in adulthood, compared with 77% of the ADHD patients. A score > or =12 on the Epworth Sleepiness Scale (usually regarded to indicate excessive daytime sleepiness) was found in 37.7% of the ADHD patients compared 95.9% of the hypersomnia patients. In ADHD patients, inattention scores correlated with the excessive daytime sleepiness score. We conclude that one should be aware of possible diagnostic confusion between narcolepsy or IH and adult ADHD when using self-report questionnaires. The high percentage of symptom overlap found in our study raises questions about possible misdiagnosing of both conditions, comorbidity with sleep problems in adult ADHD, and the validation of the used scales. It remains unclear whether our findings indicate pathophysiological overlap.