Purpose We aimed to establish whether PD-1 and PD-L1 expression, in ovarian cancer (OC) tumour tissue and blood, could be used as biomarkers for discrimination of tumour histology and prognosis of OC. Experimental Design Immune cells were separated from blood, ascites and tumour tissue obtained from women with suspected OC and studied for the differential expression of possible immune biomarkers using flow cytometry. PD-L1 expression on tumour associated inflammatory cells was assessed by immunohistochemistry and tissue microarray. Plasma soluble PD-L1 was measured using sandwich ELISA. The relationships among immune markers were explored using hierarchical cluster analyses. Results Biomarkers from the discovery cohort that associated with PD-L1+ cells were found. PD-L1+ CD14+ cells and PD-L1+ CD11c+ cells in the monocyte gate showed a distinct expression pattern when comparing benign tumours and epithelial ovarian cancers (EOC) - confirmed in the validation cohort. Receiver operating characteristic curves showed PD-L1+ and PD-L1+ CD14+ cells in the monocyte gate performed better than the well-established tumour marker CA-125 alone. Plasma soluble PD-L1 was elevated in EOC patients compared to healthy women and patients with benign ovarian tumours. Low total PD-1+ expression on lymphocytes was associated with improved survival. Conclusions Differential expression of immunological markers relating to the PD-1/PD-L1 pathway in blood can be used as potential diagnostic and prognostic markers in EOC. These data have implications for the development and trial of anti PD-1/PD-L1 therapy in ovarian cancer.