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).