Abstract: Objective To determine levels of undetected dementia in five provinces of China and analyze their determinants.Methods With the protocols for the 10/66 dementia research group,6 071 people(≥60 years old)in five provinces of China were investigated.Results A total of 349 cases of dementia were identified.The level of undiagnosed dementia in five provinces of China was 93.98%(328/349).Compared the diagnosed group with the undiagnosed group,there were statistical significances in education level,place of residence,the distance of the most close relatives,frequency of contacting with friends,getting the help needed from others timely,thyroid function,hearing function,activity of daily life(ADL) score,and family history of mental illness(P<0.05 or P<0.01).Multivariate logistic analyses showed that low education level(odds ratioOR=6.33),help available when needed(OR=5.1) were risk factors for the diagnosis of dementia.The long-distance residense of the most close relative or no relative(OR=0.31),the ADL score≥1(OR=0.25),and having family history of mental illness(OR=0.05)were protective factors for the diagnosis of dementia.Conclusion The elderly in five provinces had a higher level of undiagnosed dementia.Education level,social support,ADL level and family history of mental illness were determinants of undiagnosed level of dementia among the elderly in communities.The improvement of the situation requires concrete efforts of the governments and individuals.