Experts recommend that clinicians evaluating adults for attention-deficit/hyperactivity disorder (ADHD) obtain information from others who know the patient well. The authors examined correspondence between the self- and other-ratings of ADHD symptoms and impairment using 3 groups of adults recruited on the basis of their severity of ADHD: ADHD diagnosis (n = 146), clinical controls self-referring for ADHD but not diagnosed (n = 97), and community controls (n = 109). The influences of diagnostic group, informant relationship, sex of participant, IQ, and comorbid anxiety and depression on self-informant disparities were also examined. Results indicated moderate to high agreement (.59–.80) between self and others on current functioning and slightly lower levels (.53–.75) between self- and parent ratings of childhood functioning. Examination of difference scores between self- and other ratings revealed small mean disparities (−0.1 to +5.0 points) but substantial variation (SDs = –2.4 to 8.9 points) for both current and childhood ratings. Clinic referrals not diagnosed with ADHD, particularly women, had higher disparities than was evident in the ADHD and community groups. Age, IQ, and education were not associated with disparities in most ratings. Higher anxiety, in contrast, was associated with greater disparities on all current and childhood measures of both ADHD and impairment.