Eighteen geriatric patients, aged 77+/-8 years, in whom a diagnosis of dementia was formulated during an unrelated hospital stay, were studied to clarify why dementia had not been detected at an earlier stage. The control group was composed of 20 patients aged 79+/-11 years with a comparable degree of cognitive impairment who had been recognized 1-3 years previously. The index group was characterized by a lower formal education (5.2+/-3.7 years vs 8.5+/-4.7 years, p<0.05), and higher prevalence of subjects living in rural areas (50% vs 10%, p<0.006); other sociodemographic variables (age, sex, marital status, employment before retirement) could not distinguish the groups. A multivariate logistic regression analysis showed that the endpoint late diagnosis was significantly correlated with the independent variables, rural residence (odds ratio=4.65, C.I.=1.7-12.9) and lower occupational role (odds ratio=3.3 C.I.=1.2-9.5). A structured interview with relatives of the patients disclosed 3 main reasons accounting for later diagnosis : poor awareness of the problem of dementia; respect for parents and grandparents; and negligible effect of this problem. on family life and economy. In the control group, dementia had been diagnosed earlier mostly because of its heavier social and economic impact on the family.