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In the present study, it was tested whether rumination—negative, recurrent thoughts—would be associated with immune parameters and health care utilization. Because rumination has been associated with sadness and subjective sleep quality, it was tested whether these factors mediated the possible effects of rumination. A young sample and an elderly sample were included to test for age differences in the association between rumination and health-related measures.A representative sample of 196 young subjects (20 to 35 years) and 314 elderly subjects (70 to 85 years) completed questionnaire measures of rumination, sadness, and subjective sleep quality. Immune measures included leukocyte counts, lymphocyte subsets, natural killer cell activity, and T-cell proliferation. Contacts with primary care physicians were registered for 1 year through central registers.Rumination displayed a positive association with total leukocyte count, total lymphocyte count, and number of B cells among the elderly, and this was not mediated by sadness or subjective sleep quality. Rumination was also positively associated with number of telephone consultations during the follow-up for the elderly, and this was partly mediated by sadness and subjective sleep quality. Although total leukocyte counts correlated with number of telephone consultations at the follow-up, none of the immune parameters mediated the association between rumination and health care utilization. No significant associations were found for the young participants.The results suggest that rumination may be associated with health-related measures in the elderly. Thus, negative thoughts may be detrimental to health, independently of negative affect.