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Background: Cancer-related cognitive dysfunction has mostly been attributed to chemotherapy; this explanation, however, fails to account for cognitive dysfunction observed in chemotherapy-naïve patients. In a controlled, longitudinal, multisite study, we tested the hypothesis that cognitive function in breast cancer patients is affected by cancer-related post-traumatic stress.Methods: Newly diagnosed breast cancer patients and healthy control subjects, age 65 or younger, underwent three assessments within one year, including paper-and-pencil and computerized neuropsychological tests, clinical diagnostics of post-traumatic stress disorder (PTSD), and self-reported cognitive function. Analysis of variance was used to compare three groups of participants-patients who did or did not receive chemotherapy and healthy control subjects-on age- and education-corrected cognitive performance and cognitive change. Differences that were statistically significant after correction for false discovery rate were investigated with linear mixed-effects models and mediation models. All statistical tests were two-sided.Results: Of 226 participants (166 patients and 60 control subjects), 206 completed all assessment sessions (attrition: 8.8%). Patients demonstrated overall cognitive decline (group*time effect on composite z -score: -0.13, P = .04) and scored consistently worse on Go/Nogo errors. The latter effect was mediated by PTSD symptoms (mediation effect: B = 0.15, 95% confidence interval = 0.02 to 0.38). Only chemotherapy patients showed declined reaction time on a computerized alertness test. Overall cognitive performance correlated with self-reported cognitive problems at one year ( T = -0.11, P = .02).Conclusions: Largely irrespective of chemotherapy, breast cancer patients may encounter very subtle cognitive dysfunction, part of which is mediated by cancer-related post-traumatic stress. Further factors other than treatment side effects remain to be investigated. [ABSTRACT FROM AUTHOR]