To determine the specificity of suggested endophenotypes of pediatric bipolar disorder (PBD), the performance of 15 euthymic children with PBD was contrasted with that of 20 children with attention-deficit/hyperactivity disorder (ADHD), a population with reportedly similar executive dysfunction, and 18 children with both PBD and ADHD. Children with PBD and PBD+ADHD (ages 8 to 17) demonstrated higher intraindividual variability in reaction time, slower processing speed, and more sluggish motor preparedness than did children with ADHD. The findings support the contention that processing speed, intraindividual variability, and slower and more variable reaction time as interstimulus interval lengthens are likely specific endophenotypes of PBD.