A documented diagnosis only benefits patients who know about it. Using nationally representative Health and Retirement Study data linked to Medicare claims (1998–2020), we quantify the gap between clinically documented dementia diagnoses and patients' own reports. Among self-respondents with probable dementia and a claims-based diagnosis, 67 percent do not report having been diagnosed—more than double the average underreporting rate for arthritis, hypertension, diabetes, and depression among the same population—and underreporting is highest in the early disease stage, precisely when decision-making capacity is greatest. Underreporting is more prevalent among individuals who live alone, are dually eligible, have less education, and are non-Hispanic Black, and less prevalent among Medicare Advantage enrollees and patients seen by dementia specialists, consistent with roles for stigma, social vulnerability, and provider disclosure incentives. Underreporting predicts lower post-diagnosis care engagement and a lower likelihood of establishing a will or trust, suggesting information frictions undermine the returns to early detection.