Emotional lability is recognized as an associated feature of attention-deficit/hyperactivity disorder (ADHD). However, the degree of phenotypic and etiologic overlap between emotional lability and the ADHD dimensions of hyperactivity-impulsivity and inattention remains unclear. The present study examines these associations in a large, community twin sample.Structural equation models were fit to data from 1,920 child and adolescent twin pairs (age range, 5-18 years). Symptoms of hyperactivity-impulsivity (HI) and inattention (IA) were assessed using a modified version of the DuPaul rating scale, completed by parents. Symptoms of emotional lability (EL) were assessed using the parent-rated Conners 10-item scale.There were moderate to strong phenotypic correlations between HI, IA, and EL. Multivariate twin modeling revealed that a common pathway model best accounted for the covariance among these dimensions, represented by a highly heritable latent factor. Ad hoc analyses confirmed that all additive genetic influences on HI, IA, and EL were shared, and identified a significantly stronger association of EL with the latent ADHD factor in older than in younger individuals.Emotional lability was phenotypically and genetically associated with hyperactivity-impulsivity and inattention in children and adolescents. The finding that a single, heritable, latent factor accounted for covariation among these phenotypes indicates that their co-occurrence is primarily the result of overlapping genetic effects. These data support the hypothesis that emotional lability is etiologically relevant to the core ADHD phenotype, and that it should be targeted in assessment and treatment in clinical practice.