Updated publication reference for PubMed record(s): 30760538. High-grade ovarian cancer (HGOC) is the leading cause of mortality from gynecological malignancies, due to diagnosis at a metastatic stage. Current screening options fail to improve mortality due to the absence of early-stage-specific biomarkers. We postulated that a liquid biopsy, such as utero-tubal lavage (UtL), may identify localized lesions better than systemic approaches of serum/plasma analysis. Furthermore, while mutation-based assays are challenged by the rarity of tumor DNA within non-mutated DNA, analyzing the proteomic profile, is expected to enable earlier detection, as it reveals perturbations in both the tumor as well as in its microenvironment. To attain deep proteomic coverage and overcome the high dynamic range of this body fluid, we applied our method for microvesicle proteomics to the UtL samples. Liquid biopsies from HGOC patients (n=49), controls (n=127) and healthy BRCA mutation carriers (n=25), were divided into a discovery and validation sets. Data-dependent analysis of the samples on the Q-Exactive mass spectrometer provided depth of 8,578 UtL proteins in total, and on average ~3,000 proteins per sample. We used support vector machine algorithms for sample classification, and crossed three feature-selection algorithms, to construct and validate a 9-protein classifier with 70% sensitivity and 76.2% specificity. The signature correctly identified all Stage I lesions and highlighted increased risk in healthy BRCA carriers. These results demonstrate the potential power of microvesicle-based proteomic biomarkers for early cancer diagnosis.