ConstitutionalBRCA1-methylation is a cancer risk factor for breast (BC) and ovarian (OC) cancer. MiR-155, regulated byBRCA1, is a multifunctional microRNA that plays a crucial role in the immune system. The present study assessed the modulation of miR-155-5p expression in peripheral white blood cells (WBCs) of BC and OC patients and cancer-free (CF)BRCA1-methylation female carriers. Additionally, we investigated the potential of curcumin to suppress miR-155-5p inBRCA1-deficient breast cancer cell lines. MiR-155-5p expression was measured using a stem-loop RT-qPCR method. Gene expression levels were determined using qRT-PCR and immunoblotting. MiR-155-5p was more highly expressed in theBRCA1-hypermethylated HCC-38 and UACC-3199 BC cell lines than in theBRCA1-mutated (HCC-1937) and WTBRCA1(MDA-MB-321) cell lines. Curcumin suppressed miR-155-5p in the HCC-38 cells but not in the HCC-1937 cells via the re-expression ofBRCA1. Elevated levels of miR-155-5p were detected in patients with non-aggressive and localized breast tumors and in patients with late-stage aggressive ovarian tumors, as well as in CFBRCA1-methylation carriers. Notably, IL2RG levels were reduced in the OC and CF groups but not in the BC group. Together, our findings suggest opposing effects of WBC miR-155-5p, according to the cell and cancer type. In addition, the results point to miR-155-5p as a candidate biomarker of cancer risk among CF-BRCA1-methylation carriers.