The aim of the present study was to compare the characteristics of patients referred to our heart failure outpatient clinic with those of patients enrolled in clinical trials on heart failure pharmacological treatment. Thus, we estimated the proportion of patients admitted to our heart failure outpatient clinic who would have been included in randomized controlled trials evaluating the effects of medical treatments on heart failure mortality, published over a 10 years period (1993-2003). Sixteen studies (n = 45276) and 299 consecutive outpatients, were included. On average, only 34% of the outpatients would have been included in at least one of the 16 trials (8-71%). The main reasons for exclusion were: NYHA class (70% were in NYHA class II), ejection fraction (29% had EF > 35%), co-morbidity (51% had co-morbidity, mainly renal failure, COPD, and disthyroidism), age (22% were older than 80 years), and occurrence of a recent acute event (50% experienced an ischemic coronary syndrome, revascularization, pulmonary edema, or stroke in the prior 6 months). These results underline the crucial role of patient selection in clinical trials, raising uncertainties about the complete applicability of trial results to clinical practice.