A few studies have been carried out in humans on the morphology and function of the cardiovascular system in the early phases of hypertension. Furthermore, the data which have been provided are controversal and, in particular, a hyperkinetic status has been found only by some authors. For this reason, we planned to study the type and degree of cardiac involvement in 25 male adolescents with mild hypertension, by means of M-mode echocardiography. The control group consisted of 100 normotensive males matched for age and sex. The following indices of left ventricular hypertrophy were significantly higher in cases than in controls: posterior wall thickness (10.4 +/- 1.0 vs 9.1 +/- 1.7 mm; p less than 0.001), cross sectional area (18.3 +/- 2.7 vs 16.6 +/- 3.2 cm2; p less than 0.02) and left ventricular mass (230.9 +/- 46.7 vs 206.2 +/- 54.0 g; p less than 0.05). After normalizing for body surface area, only posterior wall thickness was still significantly greater in cases (5.7 +/- 0.6 vs 5.2 +/- 1.0 mm/m2; p less than 0.01). Among the indices of left ventricular function, only the posterior wall excursion was significantly greater in mild hypertensives than in controls. Our data do not confirm the existence of a hyperkinetic status in subjects with mild hypertension. The type of morphological alterations we found and their modest correlation to blood pressure suggest that factors besides an increased afterload are involved in the development of left ventricular hypertrophy.