Forty-eight children and adolescents (mean age 10.5 years, range 1.25-18 years) with clinical evidence of Marfan syndrome were studied. Height and weight percentiles were established. Cardiac dimensions and morphology were studied by M-mode and 2D-echocardiography. At diagnosis left atrial and left ventricular end-diastolic diameter and left ventricular posterior wall thickness were within normal limits except in a few adolescent patients. Interventricular septum was thickened in about 20% and aortic diameter increased in 56% of the patients. An additional 13% of patients developed aortic dilation during the study period. At diagnosis regression analysis revealed a significant (P less than 0.05) correlation of the aortic diameter, septal thickness and the posterior left ventricular wall thickness and body surface area. Follow up studies of 19 patients allowed documentation of the development of aortic root dilation.