Reporting source bias was examined using cognitive data from a probability sample of adult day health care (ADHC) clients. Data were obtained from four sources: client, family, staff, and chart. These data suggest that prevalence estimates are influenced by the reporting source and method of assessment, with chart diagnoses yielding the least convergent estimates. Examination of agreement summary statistics and bias indices show that both staff and family underreport prevalence of cognitive impairment, but that more bias is associated with staff than is with family sources. Such bias should be considered in studies of prevalence estimation of cognitive impairment.