Twenty-two adults with mild concussions were assessed 5 times during the first 3 months after injury. The initial tests were performed within 72 hours of injury. Each evaluation included a neurological examination and neuropsychological reaction time (RT) tests of simple and choice RT variations. The concussed subjects were compared with control subjects matched for age, sex, and education. The time of day of the testing was equated for the two groups. None of the concussed subjects had a significant neurological deficit and none was hospitalized. There was no significant difference in the number of errors by the two groups on the RT tests. On the simple RT test, requiring a predetermined response to a specific signal, there was no significant difference between the groups, although the concussed group was approximately 28 ms slower on the average than the control group. On the choice RT tests, however, which demand an increased amount of attention and information processing, the concussed subjects were significantly slower than the normal control group, especially during the 1st month after injury. Even after 3 months, the concussed subjects had not yet attained the skill of the control group. Analysis of the response curves over time suggested two processes: an improvement in the concussed group and a slowing in the control group. Within the concussed group, there was no correlation of RT with the severity of the concussion. Even mild concussions can cause significant attentional and information processing impairment in the absence of any apparent neurological problems. Specific neuropsychological tests are necessary to reveal the deficit. A significant impairment seems to last for several weeks.(ABSTRACT TRUNCATED AT 250 WORDS)