Thirty-one patients with cystic fibrosis of varying severity were examined by echocardiography. Right ventricular dimension (RVD) was above upper normal limit in 14 patients and right ventricular dimension index (RVD index) was higher than the upper normal limit in 11 patients. Furthermore, there was a significant relationship between increasing RVD index and 1) decreasing forced vital capacity (FVC) both actual test results and average 6 months values; and 2) decreasing peak-expiratory flow rate (PEFR) both actual test results and average 6 months values. This observation suggests a persistent heart involvement. Five patients had either heart failure and/or electrocardiographic evidence of right ventricular abnormality. These patients had increased RVD index and one patient with the highest RVD index died 8 weeks after the examination. The present study has shown the usefulness of echocardiographic measurement of right ventricular dimension and of septal motion in assessing cor pulmonale, before development of electrocardiographic abnormalities and right heart failure.