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Prostate cancer screening with prostate specific antigen reduces prostate cancer mortality but leads to over diagnosis of indolent prostate cancer. The use of 5α-reductase inhibitors lowers prostate specific antigen and in theory could affect the performance of prostate specific antigen based screening. We evaluated the outcomes of prostate cancer screening in 5α-reductase inhibitors users.The study was performed in FinRSPC (Finnish Randomized Study of Screening for Prostate Cancer). Of 80,454 men 31,866 were randomized to be screened at 4-year intervals during 1996 to 2004. Information on 5α-reductase inhibitors reimbursements before prostate cancer during 1995 to 2009 was collected from the national prescription database for 78,615 men. We evaluated the effect of screening on prostate cancer risk and mortality by 5α-reductase inhibitors using Cox regression.Men receiving 5α-reductase inhibitors had higher median prostate specific antigen and were more often screen positive than nonusers. Despite this, screening did not significantly affect prostate cancer detection (HR 0.89, 95% CI 0.79–1.01) or mortality (HR 0.82, 95% CI 0.51–1.32) compared to findings in the control arm among men on 5α-reductase inhibitors. On ROC analysis prostate specific antigen and age did not predict Gleason 7-10 prostate cancer as accurately in 5α-reductase inhibitors users as it did among nonusers (first screening round AUC 0.79 vs 0.88).Prostate specific antigen based screening among men receiving 5α-reductase inhibitors did not improve the detection of high grade or metastatic prostate cancer, or prevent prostate cancer death.