The goal of the study was to examine the relationships among fatigue catastrophizing, core dysfunctional beliefs, and fatigue in breast cancer radiotherapy patients. Seventy-eight patients participated in the study (mean age = 56.3, standard deviation = 10.5). Patients completed questionnaires on fatigue catastrophizing, core dysfunctional beliefs, and fatigue in their last week of radiotherapy. Using bootstrapping procedures to obtain estimates and confidence intervals for indirect effects, results showed that core beliefs (Need for Comfort and Demandingness for Fairness) had significant indirect effects on fatigue through fatigue catastrophizing, as indicated by the 95 percent confidence interval (.02–.19 for Need for Comfort; .01–.16 for Demandingness for Fairness).