To assess whether CT attenuation values help in differentiating benign from malignant etiology of focal (18) F-FDG avid breast lesions detected on whole-body PET/CT exam in postoperative breast cancer patients. Institutional review board approval and waived informed consent were obtained for this HIPAA-compliant retrospective study. Between January 2009 and July 2011, a total of 85 patients had 97 focal (18) F-FDG avid breast lesions on whole-body PET/CT. Of these, 54 (56%) lesions were biopsy-proven primary invasive breast carcinoma that had not undergone treatment at the time of PET/CT, 35 (36%) were benign lesions, and 8 were locally recurrent breast carcinoma. Mean attenuation values were retrospectively measured in Hounsfield units (HU) for the correlative lesion on the CT portion of the exam. Receiver-operating characteristic curves (ROC) were calculated to determine the optimal cutoff values of HU that would best discriminate between benign and malignant lesions. Interobserver agreement for measured mean attenuation values was assessed by calculating the intraclass correlation coefficient (ICC). Mean HU for the benign lesions group and the local recurrence lesions group was -11.0 ± 30.3 versus 32.9 ± 6.87 (p < 0.0002). ROC curve analysis comparing benign breast lesions to local recurrence lesions found an optimal cutoff value of 17 HU (area under curve = 0.982, p < 0.0001, Sensitivity = 100%, Specificity = 89%). ICC with regard to interobserver agreement in measuring the mean HU of the benign lesions was 0.84 (95% confidence interval 0.64-0.93) and for the malignant lesions was 0.88 (95% confidence interval 0.77-0.94). A CT attenuation threshold value of less than 17 HU suggests benign etiology of focal (18) FDG avid breast lesions in postoperative breast cancer patients. If confirmed by additional studies, these findings may provide additional information to guide the treating physician regarding decisions for supplementary imaging or the need to biopsy.