Impaired coronary reserve (CR) with angiographic coronary arteries has been demonstrated in patients with left ventricular hypertrophy (LVH) in response to valvular heart disease or hypertension. To determine if adaptive LVH induced by intensive training may alter myocardial blood flow (MBF) and CR, 8 highly trained endurance tri- athletes (29.6 +/- 4.0 yrs, with echographic LVH) were compared with 6 control subjects (33.0 +/- 7.9 yrs, with a normal echographic examination). Triathletes entered the study if they had a left ventricular mass >120 g/m2 at 2-D echocardiographic measurements (mean = 148.6 +/- 19.8 g/m2). MBF was assessed using positron emission tomography (PET) with H2O15. Subjects underwent an intravenous bolus of 17- 25 mCi of H2)15 at baseline and after intravenous infusion of 0.80 mg/kg of dipyridimole; H2O15 examination was followed by an F18-fluorodeoxyglucose (FDG) myocardial imaging. CR was determined as the ration of maximal to basal myocardial blood flow. In comparison with controls, triathletes with LVH showed normal MBF values (0.74 +/- 0.1 vs 0.8 +/- 0.2 ml/ml/min, p = 0.2), but an increased CR (3.8 +/- 0.7 vs 6.1 +/-1.9, p < 0.05). In contrast with other forms of LVH, CR is not altered in LVH due intense physical training. These results suggest that LVH due to intensive physical training is associated with an increase in coronary blood flow capacity.