BackgroundLittle is known about time perception, its putative role as cognitive endophenotype, and its neuroanatomical underpinnings in adults with attention deficit hyperactivity disorder (ADHD).MethodTwenty adults with ADHD, 20 unaffected first-degree relatives and 20 typically developing controls matched for age and gender undertook structural magnetic resonance imaging scans. Voxel-based morphometry with DARTEL was performed to obtain regional grey-matter volumes. Temporal processing was investigated as a putative cognitive endophenotype using a temporal reproduction paradigm. General linear modelling was employed to examine the relationship between temporal reproduction performances and grey-matter volumes.ResultsADHD participants were impaired in temporal reproduction and unaffected first-degree relatives performed in between their ADHD probands and typically developing controls. Increased grey-matter volume in the cerebellum was associated with poorer temporal reproduction performance.ConclusionsAdults with ADHD are impaired in time reproduction. Performances of the unaffected first-degree relatives are in between ADHD relatives and controls, suggesting that time reproduction might be a cognitive endophenotype for adult ADHD. The cerebellum is involved in time reproduction and might play a role in driving time performances.