Parkinson's disease (PD) is one of the most commonly occurring neurodegenerative disorders, with lifetime incidence between 1 and 2% among people older than 65 years. ED is one of the more disabling and poorly addressed aspects of PD. The purpose of this study was to assess the efficacy and safety of sildenafil citrate in Parkinson-emergent ED. Sexual function of participants was assessed using responses to the 15-question International Index of Erectile Function (IIEF), Sexual Encounter Profile (SEP) diary questions 2 and 3, Erectile Dysfunction Inventory of Treatment Satisfaction (EDITS) questionnaire and a Global Efficacy Question 'Has the treatment you have been taking over the study period improved your erections?' This randomized, double-blind, placebo-controlled study, comprised a screening period of at least 1 month, a placebo-lead in period of 1 week and treatment period. Two hundred thirty-six patients entered the trial. These patients had mild-to-moderate PD (stages I-III Hoehn-Yahr) and were experiencing Parkinson-emergent neurogenic ED. They were randomized to receive 100 mg sildenafil on demand 1 h before sexual activity (group 1, n=118), or similar regimen of placebo (group 2, n=118). Patients were instructed to use at least 24 doses/attempts at home. At the end of the trial, differences between sildenafil and placebo groups were significant for the IIEF erectile function (EF) score (22.6 ± 4.6 vs 14.8 ± 4.2, P=0.01), for percent Global Efficacy Question 'Yes' responses (68.1 ± 4.6 vs 12.2 ± 3.2, P=0.001), for SEP2 'Yes' responses (68.1 ± 4.2 vs 32.5 ± 2.2, P=0.003), for SEP3 'Yes' responses (75.9 ± 5.4 vs 33.5 ± 4.4, P=0.004) and for mean EDITS score (69.8 ± 4.2 vs 13.0 ± 2.7, P=0.004). A normal EF domain score (>or=26) at end point was achieved by 56.9 and 8.7% of the patients in the sildenafil and placebo groups, respectively (P=0.001). Sildenafil can be considered as an effective treatment in patients with Parkinson-emergent ED.