We compared the performance of 15 adults with attention deficit hyperactivity disorder (ADHD) and a group of 16 control adults on a temporal bisection task in auditory and visual modalities. The point of subjective equality (PSE) and the difference limen (DL) were computed to analyse performance. The main findings were that (a) individuals with ADHD overestimated the duration of both the auditory and visual stimuli in comparison to the control group, as evidenced by a shift in their mean PSE; (b) individuals with ADHD also showed less precision in their estimates than did the control group as evidenced by flatter psychometric functions; and (c) the degrees of overestimation and imprecision in subjects with ADHD were comparable across modalities. These results, discussed in the framework of the pacemaker–counter clock model of time estimation, suggest that temporal difficulties encountered by ADHD patients might be explained both by an alertness effect at the level of the switch that directs pulses into the accumulator and also by distortions of durations stored in reference memory.