Detailed invasive haemodynamic studies were performed in 27 of 32 patients with severe tetanus. Nineteen had severe uncomplicated tetanus and eight had associated major complications, chiefly infection and pulmonary complications. The results were compared with those obtained from 15 healthy male volunteers who served as controls. There were two deaths in 32 patients (mortality 6.25 per cent). Severe tetanus without major complications was characterized by a high output hyperkinetic circulatory state with tachycardia (heart rate 131 (19.2) beats/minute), increased stroke volume index (43.1 (10.7) ml/m2), increased cardiac index (5.48 (0.94) l/min/m2) and a normal left ventricular stroke work index (60.5 (15.9) g/m/m2). Volume loading demonstrated a significant haemodynamic response and increased vascular capacitance. Even so the maximum percent rise from baseline values of these indices after volume load was significantly higher in controls (p < 0.001). Autonomic cardiovascular disturbances affected both sympathetic and parasympathetic activity. Hypertension and tachycardia alternating with hypotension and bradycardia were related to sudden fluctuations in systemic vascular resistance. Our studies suggested some degree of myocardial dysfunction in patients with severe uncomplicated tetanus. The haemodynamics of severe tetanus were masked and altered by complicating infection, pneumonia, and atelectasis.