Background: The ER, PR and HER2 expression is routinely determined in breast cancer patients for prognosis and treatment decision-making. The androgen receptor (AR) is also present in the majority of breast cancer patients, offering a potential new drug target.ER expression in tumor lesions can be visualized by ¹⁸F-fluoroestradiol (FES) PET throughout the body. The AR-expression could be visualized by ¹⁸F-fluorodihydrotestosterone (FDHT) PET in prostate cancer patients.The aim was to obtain accurate non-invasive information about ER and AR tumor receptor status with FES and FDHT PET in breast cancer patients. Methods: Metastatic breast cancer patients with an ER positive primary tumor and ≥ 1 metastasis were eligible. Patients were staged with a bone scan and chest and abdominal CT scan. Lesions visible on conventional imaging were evaluable for primary endpoint, i.e. the concordance of FES and FDHT uptake with respectively ER and AR expression determined immunohistochemically (IHC) in a metastasis. Patients underwent a FES PET and FDHT PET scan within 2 weeks. ER and AR staining was performed centrally according to ASCO guidelines. IHC > 10% and > 1% nuclear staining was considered AR and ER positive, respectively. Correlations between PET and IHC were calculated using Spearman’s correlation coefficient. Results: 13/24 patients were evaluable for the primary endpoint. 5 patients had non-representative biopsies, biopsied liver lesions (n = 3) were not evaluable on PET due to high physiological background uptake in healthy tissue. 2 skin lesions and 1 intestinal lesion were not visible on conventional imaging. The correlation between AR expression and FDHT uptake (SUV_max ) was ρ: 0.67 (p = 0.01) and between ER expression and FES uptake was ρ: 0.75(p = 0.003). Conclusions: It is feasible to visualize AR expressing lesions with FDHT PET in metastatic breast cancer patients, and FDHT uptake correlates with AR expression in representative biopsies. FES uptake shows a similar correlation with ER expression. Supported by CTMM-Mammoth project. Clinical trial information: NCT01988324