Neurologists' use of ICD-9CM codes in a sample of 181 consecutive, new patients evaluated for dementia by 48 of all 49 neurologists from the Rochester, NY, area over a 1-year period was examined. The specific code for AD, 331.0, was used for only 36.5% of patients judged by the neurologist to have AD as the most likely diagnosis. Other codes used were not inaccurate but would result in lower reimbursement. Variation in coding could affect validity of dementia research using claims data.