['Although numerous social factors have been associated with cognition in older adults, these findings have been limited by the consideration of individual factors in isolation. We investigated whether social vulnerability, defined as an index comprising many social factors, is associated with cognitive decline.', 'In this secondary analysis of the Canadian Study of Health and Aging, 2468 community-dwellers aged 70 and older were followed up for 5 years. The social vulnerability index incorporated 40 social variables. Each response was scored as 0 if the "deficit" was absent and 1 if it was present; the 40 deficit scores were then summed. For some analyses, index scores were split into tertiles of high, intermediate, and low social vulnerability. Cognitive decline was defined as a >or=5-point decline in the Modified Mini-Mental State Examination (3MS). Associations of social vulnerability with 5-year cognitive decline (adjusting for age, sex, frailty, and baseline cognition) were analyzed by using logistic regression.', 'Mean social vulnerability was 0.25 (standard deviation, 0.09) or 9.9 deficits of the list of 40. The median cognitive change of -1.0 (interquartile range, -6 to 2) points on the 3MS was noted at 5 years. About 743 individuals (30% of the sample) experienced a decline of >or=5 points on the 3MS. Each additional social deficit was associated with increased odds of cognitive decline (odds ratio, 1.03; 95% confidence interval, 1.00 to 1.06; P = .02). Compared with those with low social vulnerability, individuals with high social vulnerability had a 36% increased odds of experiencing cognitive decline (odds ratio, 1.36; 95% confidence interval, 1.06 to 1.74; P = .015).'] Does social vulnerability predict cognitive decline in a prospective cohort of older Canadians?