Background and Purpose: Prospective use of the CEM43 degrees CT90 thermal dose parameter has been proposed for hyperthermia treatments. This study evaluates the CEM43 degrees CT90 parameter by means of a retrospective analysis of recurrent breast cancer patients receiving reirradiation plus hyperthermia. Material and Methods: CEM43 degrees CT90 was calculated for 72 patients who received 8 x 4 Gy reirradiation plus 8 x 1 h hyperthermia for adenocarcinoma recurrences at the chest wall. Associations of prognostic factors CEM43 degrees CT90 and tumor maximum diameter with endpoints complete response (CR), duration of local control (DLC), and overall survival (OS) were determined. Results: A highly significant inverse association between CEM43 degrees CT90 and tumor maximum diameter (p = -0.7, p < le-6) was found. The association between CR and CEM43 degrees CT90 was not significant (p >= 0.7). CEM43 degrees CT90 was associated with DLC. Both CEM43 degrees CT90 and tumor maximum diameter had a significant association with survival (p <= 0.01). The association with thermal dose, when adjusted for tumor maximum diameter, was not significant for either CR, DLC, or OS (p > 0.2). Conclusion: In this retrospective study, no clear CEM43 degrees CT90 thermal dose targets or associations with clinical endpoints could be established.