Objective: Symptoms of overactivity form part of the "DSM-IV" criteria for the combined or hyperactive-impulsive subtypes of attention-deficit/hyperactivity disorder (ADHD); yet little data exist that would quantify the nature of the overactivity component. We aimed to quantify the ability of four different measures of motion sensor data, taken from actigraphs, and the intraindividual variability (IIV) in these measures, to distinguish ADHD cases from controls. Furthermore, we aimed to investigate the degree of shared familial influences on these measures and the ADHD diagnosis. Method: Receiver operating characteristic analysis and multivariate structural equation modeling were used on actigraph data collected during a cognitive testing session in a sample of 116 ADHD combined-type probands, 119 of their siblings, and 218 control siblings (age range 6-18 years). Results: Three measures of actigraph data--the number of movements made, the magnitude of these movements, and the IIV in the magnitude of movements--yielded an area under the curve of up to 0.8, indicating an ability to distinguish between cases and controls. The latter two of these measures showed significant shared familial vulnerability with an ADHD diagnosis, with high ADHD-actigraph familial correlations. Conclusions: The actigraph data support the "DSM-IV" conceptualization of including overactivity as one of the core features within ADHD combined subtype. The magnitude of movements made, and the IIV of these movements, may be suitable candidates for future molecular genetic studies seeking to identify polymorphisms associated with the risk for ADHD. Further research should investigate if these findings generalize to a more naturalistic, homelike setting. (Contains 4 tables and 1 figure.)