ADHD is often associated with symptoms of emotional lability (EL), such as irritability, hot temper, low frustration tolerance, and sudden unpredictable shifts towards negative emotions such as anger, dysphoria and sadness. EL has been consistently noted in the changing definitions of the ADHD construct and its diagnosis and is currently considered an associated feature of the disorder. However, it is still unclear how EL and ADHD are interconnected. Thus, 1186 children with ADHD-combined type and 1827 siblings (aged 6-18 years) were assessed for symptoms of EL, ADHD, associated psychopathology and comorbid psychiatric disorders with a structured diagnostic interview as well as parent and teacher ratings of psychopathology. EL was associated with increased severity of ADHD core symptoms, particularly hyperactivity-impulsivity, and more symptoms of comorbid psychopathology, primarily symptoms of oppositional defiant disorder (ODD), but also affective symptoms, and substance abuse. However, EL was only partly explainable by the severity of ADHD core symptoms and associated psychopathology, and seems to be more closely related to oppositional symptoms. Severity of EL in probands increased severity of EL in siblings but not prevalence rates of ADHD or ODD.