Background: Patients (pts) with HER2-negative (HER2-) advanced metastatic breast cancer (MBC) may present with elevated serum levels of the HER2 extracellular domain (sHER2) or HER2 overexpressing circulating tumor cells (CTCs), potentially qualifying for anti-HER2 therapy. This retrospective study sought to clarify the value of HER2-directed Tx in ´occult´ HER2-positive (HER2+) MBC pts in the clinical routine. Methods: 31 pts with HER2- MBC (ER+, n = 27) exhibiting sHER2 values > 15 ng/mL (n = 8), HER2+ CTCs (n = 7), or both (n = 16) were included. Pts had failed 2-16 prior systemic treatments (median: 7). Pts received trastuzumab (T: n = 19), lapatinib (L: n = 4), T+L (n = 2), or T+pertuzumab (TP: n = 6). Anti-HER2 Tx was given alone (n = 5), or in combination with endocrine agents (n = 6), cytotoxics (n = 17), or other targeted drugs (n = 3). Responses were scored according to RECIST 1.1, OS was calculated from the start of Tx until death from any reason or loss to follow-up. Results: Anti-HER2 Tx was well tolerated. Median treatment duration was 17.0 wks. In 2 pts with L and 1 pt with T+L, Tx was discontinued due to toxicity (diarrhea, fatigue). 12 PR, 13 SD, and 6 PD accounted for an objective response rate of 35.4% and a clinical benefit rate of 80.6%. Median OS was 76.1 wks. In 25 pts, 9 with PR, 12 with SD, and 4 with PD, sHER2 measurements at baseline and after 3 wks were available. Most pts with PD showed increasing sHER2 levels. In the majority of pts with PR or SD, sHER2 dropped by more than 20% from baseline. However, 2 pts with PR to L, had increasing sHER2 valued. In 20 pts, 8 with PR, 7 with SD, and 5 with PD, serial CTC counts 6 wks from baseline were available. 4 pts with PD showed increasing CTCs. All pts with SD and PR presented with declining CTCs, mostly normalizing within 6 wks. Conclusions: Anti-HER2 Tx was effective in pts with pretreated occult HER2+ MBC. Thus, determination of both sHER2 and HER2 overexpressing CTCs is reasonable in tissue HER2-negative MBC pts in the clinical routine. Compared to sHER2, serial CTC measurements may be the more accurate predictor of response to anti-HER2 treatment, particularly in pts receiving L as part of their Tx.