Plasmodium falciparum malaria continues to threaten human populations in the tropics and travellers in endemic areas. Drug resistance of the parasite is a major problem in treating this devastating disease. In a prospective trial we investigated the in vitro sensitivity of Plasmodium falciparum to chloroquine, quinine and mefloquine in the Albert Schweitzer Hospital in Lambaréné, Gabon every second year from 1992 to 1998. We used the standard WHO in vitro sensitivity assay. Parasite sensitivity to quinine and mefloquine remained stable over the years. However, parasite resistance to chloroquine decreased highly significantly with the change in local malaria treatment policy. In 1992, 100% of parasite isolates showed resistance to chloroquine, whereas in 1998 only 45% were found resistant.