A reduction of functional capacity has been reported in severe hypertension. However, the reduced peripheral vasodilation observed in the early stages of hypertension, could also impair the blood supply to exercising muscles in mild hypertensives presenting a normal left ventricular mass. In this paper the cardiopulmonary exercise capacity of early hypertensives has been investigated. Thirty mild hypertensives (9 in stage I and 21 in stage II according to WHO) and 36 normotensives divided into two age and weight-matched groups, were investigated. All subjects underwent a stress test according to the modified Bruce protocol with contemporary assessment of breath-by-breath expiratory gas analysis and measurement of the anaerobic threshold (AT) and of the oxygen consumption at peak exercise (PVO2). Exercise duration and maximal workload, in stage I hypertensives, were similar to controls but the O2 consumption was significantly reduced in comparison to controls (P = 0.043). On the contrary, in stage II patients exercise duration, maximal workload, PVO2 and AT were significantly lower than in normotensives. No relationship between myocardial hypertrophy and ergometric or ventilatory (PVO2, AT, VE) parameters was found. In conclusion an early impairment of the aerobic exercise performance is detectable in uncomplicated (stage I WHO) mild hypertensives.