To investigate whether use of dutasteride, a 5-alpha reductase inhibitor, for at least four weeks preoperatively affected the blood loss during open prostatectomy (OP). Retrospective analysis was made of the data of 110 patients who had undergone OP. Group I comprised 50 patients that used dutasteride for 4 weeks preoperatively, and Group II comprised 60 patients that did not use the drug. The groups were compared in respect of age, total prostate specific antigen (TPSA) levels, prostate volumes, preoperative hemoglobin (Hgb) and hematocrit (Hct) levels, postoperative reduction of Hgb and Hct, percentage reduction in Hgb and Hct, and the administration of postoperative blood products. No differences were determined between the two groups in respect of prostate volumes, TPSA, preoperative Hgb and Hct levels (P = .813, P = .978, P = .422, P =.183, respectively). Postoperative Hgb reduction was 2.19 ± 1.36 g/dL in Group I, and 2.5 ± 1.47 g/dL in Group II (P = .260). Hgb reduction was calculated as 16.4 ± 9.7% in Group I and 17.6 ± 9.7% in Group II (P = .505). Reductions in Hct were 5.8 ± 3.7% in Group I, and 7.3 ± 4.4% in Group II, and percent reductions were 14.8 ± 9.4% in Group I and 17.3 ± 10.2% in Group II (P = .068, P = .182, respectively). The use of dutasteride before OP did not affect blood loss during surgery, therefore surgery should not be delayed for the administration of dutasteride to patients.