The association between estrogen receptor (ER) positive breast cancer (BC) and autoimmune disorders has been recently recognized. In particular exposure to aromatase inhibitors is associated with a significant increased risk of rheumatological autoimmune disorders. The purpose of this study was to investigate Sjogren syndrome (SjS) occurrence in patients with ER-positive BC. This is a prospective study analyzing 110 consecutive patients with ER-positive BC treated with anti-hormonal therapy. New 2016 American College of Rheumatology/European League against Rheumatism (ACR-EULAR) classification criteria were used to identify patients with SjS. Ultrasonography of salivary glands (SG) was used to screen patients with negative disease biomarkers, to candidate them to SGs biopsy. Sicca syndrome was detected in 51 patients (46%), whereas a true primary SjS was diagnosed in 11 patients (10%). Even if the evaluation of incidence and prevalence of primary SjS vary widely, to the best of our knowledge, the data from the present study emphasize a previously unsuspected high prevalence of defined pSjS that causes BC sicca symptoms complaints. Hypothesis, explanation of this link and even possible biases are discussed.