Radical retropubic prostatectomy (RRP) is an important cause of iatrogenic erectile dysfunction (ED). While sildena®l has been widely used since its introduction as a new treatment option for ED, its ef®cacy in post-RRP patients has not been extensively studied. We retrospectively compared the ef®cacy of sildena®l in post-RRP and non-surgical patients with ED (NSED) using a subset of questions from the International Index of Erectile Function (IIEF) and correlated results with their speci®c etiology of ED based on penile blood¯ow study (PBFS).A brief questionnaire regarding satisfaction with sildena®l was administered to 72 consecutive post-RRP patients (nerve sparing status unknown) and 32 consecutive NSED patients who had previously undergone PBFS with pharmacotesting as part of their evaluation for ED. PBFS diagnoses were arterial insuf®ciency (AI) for peak systolic velocity (PSV)`25 cmasec; venogenic (CVOD) for PSV 35 cmasec, mixed vascular for PV b 25 but`35 cmasec and resistive index (RI)`0.9; a vascular normal diagnosis (neurogenic impotence) required excellent rigidity sustained for 20 min. Differences in the IIEF subscores for the different groups of patients were assessed. Success with sildena®l was de®ned as moderate or excellent improvement (3a4 or 4a4) with ability for penetration.No differences were found among the different subgroups of RRP patients with respect to IIEF scores or success rates with sildena®l. NSED patients had both signi®cantly higher post-treatment IIEF scores (3.6a3.4 vs 2.5a2.2; t 4.50, P`0.0001) and success rates (63% vs 31%; t 3.11, P`0.01) with sildena®l treatment than RRP patients.We found that sildena®l is signi®cantly less effective in impotent RRP patients than in agematched patients with ED (31% vs 63%). We had postulated that sildena®l would be least effective among RRP patients with excellent sustained rigidity to PGE1, as this subgroup is likely to have neurogenic impotence. We found that sildena®l response rates among subgroups of RRP patients were statistically similar regardless of PBFS diagnosis. IIEF scores for the RRP subgroups were similar but statistically lower than in men with ED and no history of RRP. While individuals with normal vascular responses to PGE1 have an increased likelihood of having neurogenic impotence, in RRP patients, we were unable to demonstrate any difference in ef®cacy of sildena®l, regardless of the PBFS diagnosis.