The Erectile Dysfunction Observational Study (EDOS) is a 6-months observational prospective multicentric study enrolling men with erectile dysfunction (ED) who asked, to be started on a treatment or to change a previous treatment. Aims of the study were to analyse the pattern of treatment and compare the efficacy of treatments used. Patients were enrolled during a normal hospital visit and were prescribed a treatment for ED. They were asked at baseline and after 3 and 6 months, to answer a set of questions from the International Index of Erectile Function, Erectile Dysfunction Inventory of Treatment Satisfaction (EDITS) and Short Form of the Psychological and Interpersonal Relationships Scale questionnaires (SF-PAIRS). Clinicians were free to prescribe any therapy for ED available in the market, and to change therapy at any time during the study. Out of 1 338 patients, available for analysis at 6 months, 624 (47%) changed their treatment during the study and 714 (53%) continued with the drug prescribed at baseline. Patients assuming tadalafil had a significantly higher probability of maintaining the same treatment compared to sildenafil or vardenafil. There was no clinically significant difference in terms of efficacy, patient satisfaction, self-confidence and spontaneity between the different inhibitors of PDE5. The 'time concerns' domain score of SF-PAIRS, was statistically better in patients assuming tadalafil. In conclusion sildenafil, vardenafil and tadalafil show similar efficacy in the clinical practice. However, patients receiving tadalafil display a lower risk to discontinue or change the treatment.