P. falciparum has reduced i n vi vo suscept ibi l it y t o art esunat e i n west ern Cambodi a as compared wit h nor t hwest ern Thai l and. Resi st ance i s charact eri zed by slow parasit e clearance i n v i vo wit hout cor respondi ng reduct ions on convent ional i n v it ro susceptibility testing. Containment measures are urgently needed. (ClinicalTrials.gov number, NCT00493363, and Current Controlled Trials number, ISRCTN64835265.)