A progressive myocardial dysfunction is known to occur in prolonged septic shock. The cases of two patients with acute meningococcal infection and Waterhouse-Friderichsen syndrome are reported. Hemodynamic measurements showed, in the early stage, the classical picture of hyperdynamic septic shock, with increased cardiac output and low systemic vascular resistances. However, rapidly progressive cardiac failure then occurred, with high filling pressures and low cardiac output. One patient went into irreversible shock. In the other the administration of inotropic agents was beneficial, with a slow but progressive improvement in left ventricular function over several weeks. Acute myocarditis has been frequently observed in patients with meningococcal infections. It is suggested that the acute myocardial failure observed in those circumstances is caused by the myocarditis.