Serum cholinesterase activities were measured in 300 patients with thermal injuries. The samples were drawn immediately upon admission and thereafter daily until the time of the patients' discharge or death. According to the burn severity a characteristic decrease was noted during the first days. The decline during the first 24 hours as well as its value (measured 24 hours after admission) was found to be correlated with the total body surface area burned. In all patients the decrease in cholinesterase activity persisted for days, but in patients with inhalation trauma this decrease was significantly greater than in patients without lung injury. It seems that serum cholinesterase activity reflects not only the presence of an inhalation trauma. The cholinesterase measurement seems to be also a good method to observe the course of illness after a burn injury. Our data suggest a correlation between cholinesterase activity and morbidity. The inhalation trauma aggravates the thermal trauma.