['To attempt to predict the success rate of sildenafil citrate in erectile dysfunction patients using penile Doppler ultrasonography (PDU) measurements of peak arterial velocity and end diastolic velocity.', 'A total of 212 patients (age range 27-76 years) with vascular pathologies were included in the study. Following a PDU test, the patients were divided into arterial insufficiency, veno-occlusive dysfunction and mixed vascular pathology groups. Subsequently, patients were given sildenafil citrate 50 mg and re-evaluated 1 month later to determine its efficacy. If it was ineffective, the dose was increased to 100 mg and patients were reassessed. Arterial insufficiency and veno-occlusive dysfunction patients were classified into mild, moderate and severe groups depending on peak systolic and end diastolic velocities.', 'The overall response rate in patients with arterial insufficiency was 74.5%, regardless of the degree of arterial insufficiency or the dose of sildenafil. The severe arterial insufficiency group had a much better response to 100 mg compared to 50 mg doses of sildenafil. Although the 50 mg sildenafil dose was effective in patients with minimal veno-occlusive dysfunction, 100 mg was better than 50 mg to achieve adequate erection in the mild and severe veno-occlusive dysfunction groups.'] Does a classification based on peak systolic velocity and end diastolic velocity predict sildenafil citrate success?