The predisposing factors, clinical presentation, and outcome of severe malaria in a Karen community living on the western border of Thailand were studied over a period of 2 years. This was an area of low malaria transmission (approximately one infection per person per year), where asymptomatic malaria is unusual. In a population of 4728 persons, who had good access to facilities for malaria diagnosis and treatment, there were 2573 cases of vivax malaria, none of whom died, and there were 5776 cases of falciparum malaria, 303 (5%) of whom had severe malaria and 11 (0.2%) of whom died-a case fatality rate of 1.9 per 1000 (95% confidence interval [CI] 1.0-3.3). The risks of developing severe malaria and dying declined steadily with age. The clinical features of severe malaria differed between children and adults. Anaemia was more common in children under 5 years old than in older children and adults, whereas the incidence of cerebral involvement increased with age. Severe malaria was 3 times (95% CI 1.4-6.2) more common in pregnant than in non-pregnant women, but was 4.2 times (95% CI 2.3-7.9) less common in patients with mixed Plasmodium falciparum and P. vivax infections than in those with P. falciparum alone, suggesting that P. vivax may attenuate the severity of P. falciparum malaria.