The aim of this retrospective analysis was to examine prevalence of undiagnosed acute illness and characteristics including neuropsychiatric symptoms associated with illness in community residing older adults with Alzheimer disease or related disorders. Subjects included 265 community residing older adults with dementia who participated in 1 of 2 interventions being tested in randomized clinical trials. Measures included a brief nursing assessment and laboratory evaluations including complete blood count, blood chemistry (Chem 7), and thyroid function tests of serum samples and culture and sensitivity tests of urine samples. Undiagnosed illness was identified according to currently published criteria. Neuropsychiatric behaviors were assessed using 21 behaviors derived from standard measures. Thirty-six percent (N=96) of patients had clinical findings indicative of undetected illness. Conditions most prevalent were bacteriuria (15%), followed by hyperglycemia (6%) and anemia (5%). The behavior most often demonstrated among those with detected illness was resisting or refusing care (66% vs. 47% for those without detected illness). Individuals with detected illness had significantly lower functional status scores [3.8 vs. 4.4, t(275)=7.01, P=0.01], lower cognitive status scores [10.5 vs. 14.4, t(275)=12.1, P<0.01], and were more likely to be prescribed psychotropic medications for behavior (41% vs. 26%, χ2=3.67, P<0.05) than those without illness. Findings suggest that challenges of diagnosing acute illness with atypical presentation must be addressed to promote quality of care and the specialized needs for this vulnerable population.