BACKGROUND: The aim of this study was to compare the clinical characteristics of patients enrolled in randomized clinical trials on congestive heart failure treatment with those of real-world patients encountered in daily clinical practice. METHODS: We searched the Cochrane review methodology, Medline and SilverPlatter databases to obtain the clinical characteristics of both patients enrolled in therapeutic clinical trials and real-world patients with heart failure. We selected 27 clinical trials, and 8 prospective epidemiological studies or registries published between 1987 and 2001 which enrolled 53,859 and 18,207 patients, respectively. RESULTS: On average, compared to real-world heart failure patients, patients enrolled in clinical trials were younger (63 +/- 10 vs 75 +/- 11 years respectively, p < 0.0001), and more likely to be male (72 vs 54% respectively, p < 0.0001). Clinical trial patients showed a lower ejection fraction (26 +/- 7 vs 38 +/- 15% respectively, p < 0.0001) but a lower prevalence of NYHA functional class III-IV (62 vs 75% respectively, p < 0.0001) than real-world patients. In clinical trial patients, the prevalence of ischemic heart disease (67 vs 42% respectively, p < 0.0001) and a history of previous myocardial infarction (62 vs 42% respectively, p < 0.0001) were higher than in real-world patients. Conversely, the prevalence of chronic atrial fibrillation (12 vs 31% respectively, p < 0.0001) and of diabetes (22 vs 24% respectively, p < 0.02) was lower in trial patients than in real-world patients. CONCLUSIONS: Our data suggest that most clinical trials on congestive heart failure, on which the guidelines for clinical practice are based, have generally included patients who are not representative of the whole spectrum of patients actually managed in clinical practice.