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Higher intelligence quotient (IQ) correlates with lower systemic inflammation, consistent with an association between lower IQ and disease risk. The present study examined the role of repetitive thought (RT) in the relationship between IQ and interleukin (IL)-6. RT is thinking attentively, repeatedly, and frequently about oneself and one's world and is characterized by valence (positive-negative), purpose (searching-solving), and total quantity (much-little).Estimated IQ and RT dimension scores were assessed at baseline in a sample of older adults (N = 120, mean age = 74 years), who thereafter had blood drawn up to 10 times semiannually (n = 799). Models were adjusted for body mass index, chronological age, and statin medication.Higher IQ was associated with lower IL-6 (γ = −0.225, SE = 0.111, p = .045). Of the RT dimensions, only more total RT predicted lower IL-6 (γ = −0.037, SE = 0.011, p = .001), an effect that was not moderated by valence or purpose. More total RT accounted for part of the effect of IQ on IL-6 (indirect effect = −0.06 [confidence interval = −0.14 to −0.002]). There was also a significant interaction between IQ and total RT (F(1,119) = 6.97, p = .009), in which more total RT was more strongly associated with lower IL-6 for people with lower IQ.Although some forms of RT such as worry may have negative health correlates for older adults, engaging in RT per se can be healthy insofar as it also encompasses planning, processing, and coping. Older adults with higher IQ were more likely to engage in RT, but those with average IQ benefitted the most with regard to a marker of systemic inflammation.