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BACKGROUND. Both the 5-item short version of the International Index of Erectile Function (IIEF-5) and the Expanded Prostate Cancer Index Composite (EPIC) have been used to assess erectile function. In this study, the authors compared various definitions of potency according to the IIEF-5 and the EPIC. METHODS. Patients with prostate cancer who had completed the IIEF-5 and the EPIC within 7 days of each other were included. The Spearman correlation coefficient (rho) was calculated to assess the relation between IIEF-5 and EPIC sexual domain scores. Concordance of potency rates by IIEF-5 and EPIC was assessed in cross-tabulations. By calculating the area under the receiver operator characteristics (ROC) curve (AUC), the authors ascertained the discriminative ability of the IIEF-5 score to identify potent men as defined by the EPIC. RESULTS. Analyzing 102 questionnaire pairs, IIEF-5 and EPIC domain scores were found to be highly correlated (rho = 0.776). EPIC sexual domain scores ≥60 had high concordance with IIEF-5 scores ≥17 (98%) and with nearly all single-item definitions of potency (≥71%). However, an EPIC sexual domain score ≥80 was a very strict definition of potency, and only 54% of patients with IIEF-5 scores ≥22 met this threshold. On the basis of ROC analysis (AUC = 0.90), an IIEF-5 score ≥20 was identified as the ideal cutoff for defining potency and corresponded with an EPIC sexual domain score ≥60. CONCLUSIONS. IIEF-5 and EPIC scores were highly correlated, but potency rates varied widely, depending on the definition of potency. The current results help with the interpretation of sexual function outcomes data in patients with prostate cancer. Cancer 2008. © 2008 American Cancer Society.