Negative expression of estrogen receptor (ER) predicts response to chemotherapy in breast cancers (BCs). ER negative cancers are those with less than 1 % of nuclear staining. Tumors with 1-10 % staining are sub-classified as "low-positive" (ER-low). HER2 negative tumors with ER low staining are considered biologically and clinically equivalent to ER negative tumors. This study investigates whether ER low expression in HER2-positive (HER2+) BCs has different clinical behavior than ER negative HER2-positive tumors. We used a sample of 171 patients with HER2+ BCs to compare risk of residual cancer after neoadjuvant chemotherapy by different ER expression strength. Patients were classified into 3 groups: ER-negative (ER <1 %); ER-low (ER <10 %, any intensity or <33 % staining, weak intensity); and ER-high (ER = 10-33 %, moderate to strong intensity or >33 %, any intensity). The risk of residual cancer in patients with ER-low tumors was similar to the risk in patients with ER-negative tumors (RR = 0.76, 95 % CI: 0.30-1.93). Conversely, patients with ER-high tumors had twice the risk of residual cancer than patients with ER-negative tumors (RR = 2.20, 95 % CI: 1.46-3.31). These findings persisted after adjusting for tumor grade, clinical tumor and lymph node stage, chemotherapy regimen, and progesterone receptor status. In this cohort of patients with HER2+ BCs, ER-low tumors had a similar pathologic response to chemotherapy as ER-negative tumors suggesting similar clinical behavior. Future research should address biological explanations to these similarities between ER negative and ER low breast cancers such as HER2 enriched phenomenon.