During operations in subtropical areas over the summer months of 2001 and 2003 the authors audited 80 patients with heat-related illness, with the intention of defining the nature and distribution of the underlying pathophysiology. Haematological, biochemical and clinical data were gathered prospectively and patients allocated to diagnostic categories on the basis of the combination of clinical findings and investigations. Four basic types of heatrelated illness could be distinguished: (1) excessive salt loss with hyponatraemic dehydration, (2) hypokalaemic alkalosis with low serum bicarbonate, (3) haemodilution associated with excessive water intake in stressed individuals, and (4) loss of normal thermoregulation, characterised by high core temperature and paradoxical cessation of sweating. Most of the patients fell clearly into a single distinct category, but there was a degree of overlap. Reduction of extracellular fluid volume was a common central mechanism. Common provoking factors identified were: gastrointestinal upset, history of previous heat intolerance (35%) environmental temperatures exceeding 45°C, short period of acclimatisation (55%), travel, sleep loss, hard physical work especially if directly preceded by a period of sleep, work in confined humid spaces (45%), and lack of additional salt intake. When several of these factors were present together admission rate over one 24-hour period reached 3% of persons at risk per day. Patients are often more ill than they appear. To reduce the incidence of heat illness during future operations the following measures are proposed:

1. Avoidance of physical exertion during the heat of the day for the first 7-10 days.

2. Progressive gentle exercise in the early morning or late evening over the same period.

3. Increase in daily salt intake to 15-20gm for the first 2-3 weeks.

4. Only sufficient water intake to relieve thirst and to ensure the flow of abundant dilute urine.