Objective: This study examined the clinical features of patients with clinical diagnoses of probable Alzheimer disease (AD), possible AD, and uncertain., Design: Case study comparing three groups of AD patients diagnosed at their initial visit to an Alzheimer outpatient clinic., Setting: Southern Illinois University School of Medicine's Center for Alzheimer Disease and Related Disorders (CADRD) assessment sites (20) in rural Illinois., Participants: 300 patients assessed at CADRD between January 1, 1994 and July 1, 2000., Measurements: Patients were given an extensive clinical battery consisting of physical and neurologic examination, mental status testing including the Mini-Mental State Exam (MMSE), Short Blessed Dementia (SBD) and Blessed Dementia Scale (ADL), medical history evaluation, and laboratory tests. Other data included age at visit, gender, and medical history variables., Results: Mean MMSE, SBD, and ADL scores differed significantly between groups (p's < 0.01). In all three cognitive tests, the uncertain group was the least impaired while the probable AD group was the most impaired. A Rasch model indicated that only the cognitive measures were useful in discriminating between the three diagnostic groups., Conclusion: In general, probable AD patients were distinguished from possible AD patients by the severity of their dementia as measured by the MMSE, ADL and SBD as well as Hachinski-Ischemic Score (HIS) scores. A Rasch model did well at predicting group membership based upon dementia measures only. The uncertain group differed from the AD groups in age and dementia severity as measured by the MMSE, ADL and SBD. Noting differences between this and previous studies, we speculate disparity may be related to differences in population ethnicity.