Hemodynamic observations in 15 cases of the Hunter-Hurler syndrome are reported. Required premedication was two to three times that usually needed. No major complications or deaths occurred. As a group, these individuals tend to have slight elevations of pulmonary artery pressures and more definite elevations of systemic arterial pressures. They generally have normal cardiac outputs and, thus, elevations of total systemic, total pulmonary, and pulmonary arteriolar resistances. In only one case was there clinical and angiocardiographic evidence of valvular involvement, that case having aortic insufficiency. In another case there was only angiocardiographic evidence of mitral insufficiency. Only one child was in congestive heart failure at the time of the study. There was no evidence of coronary artery disease. The ascending limb right ventricular pressure curve was frequently notched and the pressure rise time of the left ventricle (dp/dt) was low. Nocase had a congenital malformation of the heart.Those individuals who have normal vascular bed resistances appear to live longer than those who have elevated resistances early in life. The prognosis is also better for the male Hurler of the sex-linked variety than for those cases due to autosomal recessive inheritance.