Falciparum malaria in 104 Thai patients was treated with either Mefloquine or Fansidar and patients were examined for parasitaemia for the next 28 days. All but one of the 40 patients treated with Mefloquine were cured, but the cure rates for Fansidar were only 9 . 1% for the two-tablet regimen and 19 . 4% for the three-tablet regimen. Most failures were classified as RII. Serum sulpha levels indicated that the Fansidar was being absorbed. Immediate steps should be taken to eliminate this focus of resistance and to protect against further resistance to Fansidar by using it in combination with another effective anti-malarial and by vigorous vector control measures.