A simple and accurate test to detect early‐stage breast cancer has not been developed. Previous studies indicate that the level of human endogenous retrovirus type K (group HERV‐K(HML‐2)) transcription may be increased in human breast tumors. We hypothesized that HERV‐K(HML‐2) reactivation can serve as a biomarker for early detection of breast cancer. Serum samples were collected from women without cancer (controls) and patients with ductal carcinoma in situ (DCIS) and invasive breast cancer. ELISA assays were used to detect serum anti‐HERV‐K(HML‐2) antibody titers. RNA was extracted from sera and analyzed by real‐time RT‐PCR to quantitate the level of HERV‐K(HML‐2) mRNA. We measured significantly higher serum mRNA and serum antibody titers against HERV‐K(HML‐2) proteins in women with DCIS and stage I disease than in women without cancer. At optimized cutoffs for the antibody titers, the assay produced an area under the receiver operating characteristic curve (AUC) of 0.89 (95% confidence interval 0.77–1.00) for DCIS and of 0.95 (95% confidence interval 0.89–1.00) for invasive breast cancer. These AUCs are comparable to those observed for mammograms. We also found that serum HERV‐K(HML‐2) mRNA tended to be higher in breast cancer patients with a primary tumor who later on developed the metastatic disease than in patients who did not develop cancer metastasis. Our results show that HERV‐K(HML‐2) antibodies and mRNA are already elevated in the blood at an early stage of breast cancer, and further increase in patients who are at risk of developing a metastatic disease.