We investigated whether coronary flow reserve (CFR) can be modified by tadalafil, a long-acting phosphodiesterase 5 (PDE5) inhibitor, in patients with documented coronary artery disease (CAD). CFR was non-invasively evaluated in 12 men with a positive history for erectile dysfunction (ED) and angiographically documented CAD, in the distal portion of the left anterior descending coronary artery, free from critical stenosis, with contrast enhanced echocardiography at time zero (T0). Then, after 20 mg tadalafil was orally administered CFR measurement was repeated after 2 h (T1) and after 24 h (T2). Doppler curves suitable for the analysis were obtained in all patients (CFR feasibility: 100%). The peak diastolic velocity after adenosine infusion increased from 71.3+/-14.3 cm/s at T0 to 82.5+/-24.0 at T1 (P=NS) and to 89.5+/-21.1 at T2 (P=0.0010). CFR after tadalafil increased significantly from 2.6+/-0.3 at T0 to 3.1+/-0.7 at T1 (P=0.0078) and a further increment was found at T2 (3.5+/-0.9; P=0.0010 vs T0). Our study shows that oral administration of tadalafil exerts a long standing, potentially beneficial effect on coronary microvasculature in patients with ED.