Ninety-four patients with falciparum malaria were treated with mefloquine (1000-mg single dose) and remained hospitalized in a malaria-free area for a minimum of 28 days. There was 1 parasitologic failure (grade I resistance [RI]) for a 99% cure rate (95% confidence interval, 94.2%-99.7%). Mean parasite clearance time by thick smear was 45.7 h (SD, 11.4 h). The subject in whom therapy failed had a parasite clearance time (71 h) >2 SD above the population mean. His plasma mefloquine level 48 h after administration was lower (578 ng/mL) than the range of levels from 8 randomly selected cured subjects (834-2360 ng/mL). The IC50 to mefloquine for the recrudescent strain of the RI failure was in the upper 90th percentile of IC50 values from 30 cured subjects. These results show a high mefloquine cure rate but document the onset and mechanism of the emergence of resistance.