The importance of serum cholinesterase activity in burned patients was evaluated in relation to anaesthesia. Anaesthesia included a repeated administration of suxamethonium. Thirty-two patients with an estimated area of burn between 3 and 72% were studied during 39 anaesthetic procedures. A statistically significant inverse correlation was found between serum cholinesterase activity and the apnoea period following intravenous suxamethonium. In patients with very low enzyme activity, apnoea periods of 10 to 25 min were observed. No correlation was found between the changes in serum potassium following suxamethonium and either the serum cholinesterase activity or the changes in Pco2 and pH. The most reliable parameters in predicting a dangerous increase in serum potassium following intravenous suxamethonium were shown to be 1) the time elapsed from burn injury to anesthesia and 2) the degree of burn injury. However, abnormal reactions to suxamethonium were seen as early as 9 days following injury, and rises in serum potassium to over 6 mmol/l were observed even in patients with a total burn surface of around 8%, i.e., less than the surface of one arm.