In addition to their potential as tissue-based markers for cancer classification and prognostication, the study of microRNAs (miRNAs) in blood circulation is also of interest. In the present study, we investigated the amounts of three cancer-related miRNAs, miR-21, -141, and -221 in blood plasma of prostate cancer (PCa) patients. A cohort of 51 patients with PCa was enrolled into the study, and miRNAs were measured in two subgroups, with localized/local advanced or metastatic PCa. A group of 20 healthy individuals served as the control group. miRNAs were quantified from the total RNA fraction using 200 μl plasma and the small RNA molecule RNU1A as a control for normalizing the miRNA amounts in circulation. We found similar levels of three miRNAs in healthy subjects with median values of 0.039, 0.033 and 0.04, respectively; (p = n.s.). In the patients, the miRNA levels were higher, with miR-21 being the highest (median, 1.51). The miR-221 levels were intermediate (median, 0.71) while the miR-141 displayed the lowest levels (median, 0.051). The differences between the control group and the patients were highly significant for the miR-21 (p < 0.001; area under the curve (AUC), 88%) and -221 (p < 0.001; AUC, 83%) but not for the miR-141 (p = 0.2). In patients diagnosed with metastatic PCa, levels of all three miRNAs were significantly higher than in patients with localized/local advanced disease where the difference for the miR-141 was most pronounced (p < 0.001; AUC, 75.5%). In conclusion, analysis of miR-21, -141, and -221 in blood of PCa patients reveals varying patterns of these molecules in clinical subgroups of PCa.