Effects of sildenafil citrate on nocturnal penile tumescence and rigidity (NPTR) were evaluated among sildenafil non-responding patients with psychogenic erectile dysfunction. All patients (n=30), equally divided into groups I and II, completed four consecutive nights using the RigiScan Plus device. Sildenafil citrate (50 mg) was given in the third night in group I and in the fourth in group II, whereas a placebo was given in the remaining nights. Additional patients (n=12) receiving only a placebo served as a control group. Results of NPTR recordings revealed neither significant differences between the control and non-sildenafil nights of both test groups, nor between the corresponding values of both groups (P>0.05). On the other hand, when sildenafil citrate nights of groups I and II taken together were compared with placebo nights, a significant increase of total events duration (P<0.001), average rigidity of the tip (P<0.05) and base (P<0.01), and rigidity activity unit (RAU) and tumescence activity unit (TAU) of tip and base (P<0.001) was observed. These results suggest that performance anxiety may be responsible for failure of response during awakening.