ADHD is a common neurodevelopmental disorder that persists into adulthood in around two-thirds of cases. Current operational definitions of ADHD focus on three core domains of inattention, overactivity and impulsivity, yet it is widely recognised that there is a wider range of psychopathology that co-occurs alongside these core symptoms. Among the most prevalent of these in adults with ADHD is mood instability, characterized by shifts in mood from ‘highs’ to ‘lows’, frustration, and chronic irritability. Mood instability may often be the main presenting symptom of ADHD and may also be one of the more impairing features of the disorder. Importantly, it has been noted that treatment for ADHD with stimulants or atomoxetine often results in improvements in mood instability symptoms in the same timescale as other core symptoms of ADHD, indicating that mood instability can be considered a symptom of ADHD just as much as inattention, impulsivity and hyperactivity. Here we report on key findings that show the very high correlation between ADHD and mood instability and the response of such symptoms to treatment with stimulant medications. Further work is aimed to clarify whether the problems with regulation of emotional responses and mood states seen in ADHD arise from the same underlying processes that lead to impaired regulation of attention and impulse control.