Background. Sexual dysfunction, including erectile dysfunction, is common in patients with uraemia. Despite successful treatment of male sexual dysfunction with sildenafil in non-uraemic population, its efficacy in dialysis patients is unknown. Patients and methods. In this study, 35 male HD patients (mean age 48±12 years) and 15 male CAPD patients (mean age 44±12 years) were included. In the baseline period, haemoglobin, serum urea, and albumin, Kt/V, several hormonal parameters, Beck depression scale, and penile Doppler blood flow, (peak systolic velocity after intracavernous papaverine administration) were measured. The international index of erectile function (IIEF) form was used to evaluate erectile dysfunction. Sildenafil was given to patients with erectile dysfunction at a dose of 50-100 mg/day twice a week. Results. The percentage of erectile dysfunction was similar between patients on HD (71%) and those on CAPD (80%). Patients with erectile dysfunction were significantly older and had lower free-testosterone serum levels and penile blood flow than those without. In linear regression analysis for baseline IIEF score, penile blood flow was the only independent variable associated with erectile dysfunction. IIEF score increased to a similar extent after sildenafil treatment in both HD patients (from 8.10±5.54 to 21.70±9.61, P < 0.001) and CAPD patients (from 9.90±3.87 to 21.60±10.18, P = 0.011). Changes in IIEF scores after sildenafil treatment were associated with baseline penile blood flow as an independent variable by linear regression analysis. Adverse events observed during sildenafil treatment were dyspepsia in two patients and headache in one patient. Conclusion. The rate of erectile dysfunction is high in dialysis patients. Penile blood flow is the most important factor for predicting both the development of erectile dysfunction and the response to sildenafil therapy in such patients. Oral sildenafil is an effective, reliable, well-tolerated treatment for uraemic patients with erectile dysfunction.