Aims: It is often assumed that β-blockers, e.g. metoprolol (METO), induce erectile dysfunction (ED) in men. However, cardiovascular diseases can also induce ED and there is also the possibility that psychological factors, such as fear of the disease and side effects of the prescribed drug, may also induce ED. Thus, it is often assumed that β-blockers induce ED in a large percentage of men, but this statement is not well validated and the role of the pharmacologic effect of METO per se on the occurrence of ED is largely unknown. To get an answer we selected 114 men (age 57 ± 4.7 years) without ED but with newly diagnosed arterial hypertension, and who could be treated with METO. Methods: METO (100 mg/day) was given as a retard formulation. The hypertensive men were randomized into 3 groups. In group 1 patients were fully informed (they knew that the drug was METO and that it might induce ED). In group 2 patients were partially informed (they knew that the drug was METO, but were not informed that it might induce ED). In group 3 patients were not informed either about the drug used or about the possible occurrence of ED. The first phase of the study lasted 60 days. After 60 days the incidence of ED was 32% in group 1, 13% in group 2, and 8% in group 3 (p < 0.01). All patients with ED entered the second, cross-over, double-blind phase of the study. METO was continued at unchanged dosage, and tadalafil (20 mg) and a placebo were given to treat ED. Results: Both treatments were equally effective. Conclusion: Prejudice about the possible occurrence, i.e. the Hawthorne effect, of ED with METO facilitates the occurrence of this side effect in hypertensive men. Since the etiology of this ED is largely psychological, it is not surprising that placebo is as effective as tadalafil in reversing this side effect.