OBJECTIVE To determine the frequency of unrecognized dementia in a group of men found to have dementia by population survey, and to identify factors associated with the failure of a family informant to recognize significant memory impairment. METHODS The Honolulu-Asia Aging Study, a population-based study of dementia among elderly Japanese-American men living on the island of Oahu, Hawaii. Data for this study were from the dementia prevalence survey, 1991-1993. METHODS A total of 191 noninstitutionalized men with dementia who had a reliable family informant. METHODS Failure of family informants to recognize a problem with thinking or memory in subjects with dementia. RESULTS A total of 21% of family informants failed to recognize a problem with memory among subjects subsequently found to have dementia. Among subjects with very mild dementia, 52% of family informants failed to recognize a significant memory problem compared with 13% among more severely demented subjects. Of the subjects with dementia whose family informants did recognize a memory problem, 53% failed to receive a medical evaluation for this problem. For all family informants, increasing age, fewer years of education, less severe dementia, fewer behavioral complications, fewer functional disabilities, and better performance on certain tests of memory and language were significantly associated with the family informant's failure to recognize a problem with memory. When the family informants were wives living with husbands, less severe dementia, fewer behavioral complications, fewer functional disabilities, and intact remote memory were associated with unrecognized dementia. CONCLUSIONS Unrecognized dementia was common in our population, especially among mild cases. Cognitive screening programs for the elderly and public education policies designed to increase awareness of early signs of dementia are needed if interventions for individuals with potentially treatable dementias are to be implemented.