The authors examined whether low levels of social engagement in midlife and late life were associated with the risk of incident dementia in 2,513 Japanese-American men who have been followed since 1965 as part of the Honolulu Heart Program and the Honolulu-Asia Aging Study. In 1991, assessment of dementia began; incident dementia cases (n = 222) were diagnosed in 1994 and 1997. Social engagement was assessed in midlife (1968) and late life (1991). The relation between social engagement and dementia risk was examined using Cox proportional hazards models. No level of midlife social engagement was associated with the risk of dementia. In late life, compared with participants in the highest quartile of late-life social engagement, those in the lowest quartile had a significantly increased risk of dementia (hazard ratio = 2.34, 95% confidence interval: 1.18, 4.65). However, compared with those who were in the highest quartile of social engagement at both midlife and late life, only decreased social engagement from midlife to late life was associated with an increased risk of dementia (hazard ratio = 1.87, 95% confidence interval: 1.12, 3.13). Although low social engagement in late life is associated with risk of dementia, levels of late-life social engagement may already have been modified by the dementing process and may be associated with prodromal dementia.