Previous studies have suggested that parameters measured in the flaccid state (FS) by penile duplex Doppler ultrasound (PDDU) may predict the outcome after intracavernosal injection (ICI). The aim of this study was to evaluate the association between peak systolic velocity (PSV) and cavernosal artery diameter (CAD) in the FS with PSV and clinical assessment of erection hardness (CAEH) after ICI. Fifty patients who underwent PDDU for erectile dysfunction were prospectively enrolled. PSV and CAD were measured in FS and then correlated with PSV and CAEH after ICI of 20 μg of e1 prostaglandin (PGe1). Even though PSV in FS demonstrated a significant correlation with PSV after ICI (P = 0.04, r = 0.21), none of the cutoff values tested (10, 13 and 15 cm s-1 in FS) had a combined sensitivity and specificity >80% to predict a PSV after ICI ≥30 cm s-1. No correlation was observed between CAD in FS and PSV after ICI. Also, no association was observed between PSV in FS and development of a complete full rigid erection after ICI. Although PSV in FS statistically correlates with PSV after ICI, predictive values are low, making it non-reliable to predict accurately the response to ICI of 20 μg PGe1.