This review aims to analyse the published data on the prevalence and clinical significance of breast incidental F18-FDG uptake detected by PET or PET/CT (BIU). A comprehensive computer literature search of the PubMed/MEDLINE, Scopus and Embase databases for studies on BIU published through July 2013 was performed. Pooled prevalence and malignancy risk of BIU were calculated. The literature search revealed 42 articles, and 17 were selected. One study was excluded because of data overlap but four additional studies were found screening the references. Finally, 20 articles were included in the systematic review and 13 were eligible for a meta-analysis. The pooled prevalence of BIU on all scans was 0.4 % (95 % CI 0.23-0.61 %), the pooled prevalence on scans on female patients only was 0.82 % (95 % CI 0.51-1.2 %), the pooled risk of malignancy of BIU when further analysed was 48 % (95 % CI 38-58 %) and the pooled risk of malignancy of BIU with histological examination was 60 % (95 % CI 53-66 %). The most frequent malignant lesion detected was infiltrating ductal carcinoma. Despite being rare, the identification of BIU frequently signals the presence of an unsuspected subclinical lesion, which differs from the indicated reason for which the patient was initially scanned, and the risk of malignancy is very high.