Although the development of Attention Deficit Hyperactivity Disorder (ADHD) after traumatic brain injury (TBI) has been described, it is unknown whether children with TBI and ADHD have greater neuropsychological impairments than children with TBI alone. This study examines attention, executive functioning, and memory in children with TBI-only and TBI + ADHD. Caregivers of 82 children with severe TBI completed structured psychiatric interviews at enrollment to diagnose premorbid ADHD and one-year after injury to diagnose post-injury ADHD. Children underwent neuropsychological testing one year after injury. One memory measure significantly differentiated children with TBI-only from children with newly developed ADHD [secondary ADHD (S-ADHD)] and those with premorbid ADHD that persisted after injury [persisting ADHD (P-ADHD)]. Compared with the TBI-only group, children with TBI + ADHD had worse performance on measures of attention, executive functioning, and memory. Results reveal that in children with severe TBI, the behavioral diagnosis of ADHD is associated with more difficulty in attention, executive functioning, and memory. Additionally, results suggest greater deficits in memory skills in the S-ADHD group compared with the P-ADHD group. Although findings provide preliminary support for distinguishing P-ADHD from S-ADHD, further research is needed to investigate neuropsychological differences between these subgroups of children with severe TBI.