Retaining severely impaired individuals poses a major challenge in longitudinal studies of determinants of dementia or memory decline. In the Health and Retirement Study (HRS), participants complete direct memory assessments biennially until they are too impaired to complete the interview. Thereafter, proxy informants, typically spouses, assess the subject's memory and cognitive function using standardized instruments. Because there is no common scale for direct memory assessments and proxy assessments, proxy reports are often excluded from longitudinal analyses. The Aging, Demographics, and Memory Study (ADAMS) implemented full neuropsychological examinations on a subsample (n=856) of HRS participants, including respondents with direct or proxy cognitive assessments in the prior HRS core interview. Using data from the ADAMS, we developed an approach to estimating a dementia probability and a composite memory score on the basis of either proxy or direct assessments in HRS core interviews. The prediction model achieved a c-statistic of 94.3% for DSM diagnosed dementia in the ADAMS sample. We applied these scoring rules to HRS core sample respondents born 1923 or earlier (n=5483) for biennial assessments from 1995 to 2008. Compared with estimates excluding proxy respondents in the full cohort, incorporating information from proxy respondents increased estimated prevalence of dementia by 12 percentage points in 2008 (average age=89) and suggested accelerated rates of memory decline over time.