Right from the arrival of the displaced Karen people in Thailand, Médecins sans Frontières (MSF) identified malaria as the top priority problem. A program of patient care based on the coupled laboratory/dispensary was set up in April 1984. Immediately a system of surveillance of morbidity and mortality from malaria was set up. This study consisted of analysing data gathered over a period of five years. During this time, the displaced population increased from 9,000 to 20,000. Analysis of the trends shows a hyperendemic situation with an annual incidence rate of 1,067 per thousand in 1984. This figure was 600 per thousand in 1988. 1,500 blood smears were checked each month and the positive predictive value of clinical suspicion was 45% on average. Plasmodium falciparum represented 80% of infections. The malaria case fatality ratio over the course of the last two years of surveillance was 0.3%. Five years observation show that the fight against malaria in this region can be based on the development of curative services and laboratories.