Right ventricular anterior wall thickness was evaluated by M-mode and two-dimensional echocardiography in 48 patients with hypertrophic cardiomyopathy and in a control group of 24 individuals free from heart disease. A significant increase of right ventricular anterior wall thickness was found in patients with hypertrophic cardiomyopathy (3.5 ±1.1 mm/m^2) as compared to normals (2.1 ± 0.4 mm/m^2) (p < 0.0001). Twenty-five (52%) patients with hypertrophic cardiomyopathy had right ventricular hypertrophy as defined by right ventricular anterior wall thickness equal or greater than 3.5 mm/m^2. The degree of right ventricular hypertrophy did not correlate with the thickness of any of the left ventricular segments (septal-anterior, septal-posterior, anterolateral and posterolateral) nor with their overall mean thickness, but it correlated with mean pulmonary artery pressure (r = 0.70, p = 0.003) and right ventricular systolic pressure (r = 0.62, p = 0.01). Significant differences in right ventricular hypertrophy have not been found between asymptomatic patients and patients who had one of the following clinical features: syncope, angina, congestive heart failure and/or atrial fibrillation, but the patients with heart failure had higher values of right ventricular anterior wall thickness (4.6 ± 1.2 mm/m^2) than patients without heart failure (3.2 ± 1 mm/m^2) (p = 0.004). It is concluded that right ventricular hypertrophy is a common occurrence in patients with hypertrophic cardiomyopathy, without correlation with the degree of left ventricular hypertrophy. It does not predominate in any particular type of hypertrophic cardiomyopathy, but it predominates in patients with higher right ventricular systolic and mean pulmonary artery pressures and in those with signs of congestive heart failure.