Clinical disorders of attention are common after traumatic brain injury but objective measures of their nature and severity are lacking. We developed a standardized independent work task to be performed by patients in a controlled, distracting environment. Videotaped records of patient behavior were made and coded in 15-s intervals for on-task behavior, presence of distractors and presence of extraneous motor behavior. Interrater reliability was good to excellent across eleven testing sessions involving four patients. This evaluation system revealed clinically important differences in on-task behavior among the four patients, as well as different levels of distraction in response to external stimuli. It also showed effects of different tasks and repeated sessions. These data indicate that this evaluation system and accompanying coding process are reliable and provide preliminary evidence that they produce clinically valid results.